JP2020046519A - Image formation apparatus, image formation method and image formation program - Google Patents

Abstract

To reduce the image quality deterioration.SOLUTION: An MFP causes a timing roller to convey a recording medium at a sheet feeding speed V2 faster than a transfer speed V1 at which a transfer roller conveys the recording medium for conveying the recording medium in the first state where the length in the conveyance direction of the recording medium between the first position where the timing roller contacts the recording medium and the second position where the transfer roller contacts the recording medium is longer than the first distance, executes switching control of sequentially switching the fixation speed to the first speed faster than the transfer speed V1 and to the second speed slower than the transfer speed V1 for conveying the recording medium in the second state where the length in the conveyance direction of the recording medium between the second position and the third position where the fixation roller contacts the recording medium is longer than the second distance, and sets the fixation speed to the first speed without executing the switching control in a preparation period before the release time point t1 at which the timing roller shifts to the state of not contacting the recording medium.SELECTED DRAWING: Figure 9

Description

  The present invention relates to an image forming apparatus, an image forming method, and an image forming program, and more particularly to an image forming apparatus that corrects a tilt of a recording medium and conveys the recording medium, an image forming method executed by the image forming apparatus, and an image thereof. The present invention relates to an image forming program that causes a computer that controls an image forming apparatus to execute a forming method.

  2. Description of the Related Art In an image forming apparatus such as an MFP (Multi Function Peripheral), an image is formed on a recording medium while conveying the recording medium. This image forming apparatus uses a transfer roller that transfers a toner image from an image carrier on which a toner image is formed to a sheet, a timing roller that supplies the sheet to the transfer roller, and a fixing roller that fixes the toner image transferred to the sheet. To transport the paper. Further, there is known a technique in which a sheet between a transfer roller and a fixing roller is conveyed in a bent state in order to improve image quality of a toner image.

  For example, Japanese Patent Application Laid-Open No. 2012-226137 discloses a case in which a bending amount measuring unit that measures a bending amount between a transfer nip portion and a fixing nip portion is installed, and a print without a margin is performed at a rear end portion of a transfer material. Then, based on the result of the deflection amount measuring means, the recording medium transport speed in the fixing means is controlled so that when the rear end of the transfer material comes off the transfer nip portion, the transfer material is conveyed so that the deflection amount becomes substantially zero. An image forming apparatus that controls the speed is described.

  Japanese Patent Application Laid-Open No. 2007-304167 discloses that a sheet on which an unfixed toner image is formed is nipped and conveyed by a fixing nip formed by pressing a heating member and a pressing member together. A fixing device for fixing the toner image on the sheet, wherein in a heating device applied to the image forming apparatus, an environment detecting means for detecting or predicting information on an environment in which the image forming apparatus is installed, and sandwiched by the fixing nip A recording material information acquisition unit that acquires information about the conveyed recording material; information detected or predicted by the environment detection unit; and the fixing unit according to information acquired by the recording material information acquisition unit. An image forming apparatus comprising: a control unit that controls a driving speed of a driving source to be driven.

  In addition to transporting the paper between the transfer roller and the fixing roller in a warped state, and transporting the paper between the timing roller and the transfer roller in a warped state, to reduce the transport load of the transfer roller And it is known that the deterioration of the image quality is small.

  However, the state in which the sheet between the transfer roller and the fixing roller is bent, and the state in which the sheet between the timing roller and the transfer roller is bent, presses the transfer roller against the image carrier. A force in the opposite direction to the force is received from the paper. The force in the opposite direction increases as the paper strain increases. Therefore, when the trailing edge of the sheet has passed the timing roller, the sheet is no longer bent, and the force with which the transfer roller is pressed against the image carrier rapidly changes. There is a problem that this variation may appear as streak-like noise in the image.

JP 2012-226137 A JP 2007-304167 A

  One of the objects of the present invention is to provide an image forming apparatus in which deterioration of image quality is reduced.

  Another object of the present invention is to provide an image forming method in which deterioration of image quality is reduced.

  Still another object of the present invention is to provide an image forming program in which deterioration of image quality is reduced.

  According to an aspect of the present invention, an image forming apparatus includes a driving roller, an image forming unit that forms a toner image on an image carrier, and a toner formed on the image carrier that rotates following the driving roller. A transfer roller that transfers an image to a recording medium, and a rotation axis of the transfer roller are arranged so that a distance from the rotation axis of the drive roller can be changed, and rotation of the drive roller on a plane whose normal line is the rotation axis of the transfer roller. Biasing means for biasing the rotation axis of the transfer roller toward a side on which the drive roller is disposed with respect to a surface including the rotation axis of the transfer roller, with a line connecting the shaft and the rotation axis of the transfer roller as a normal line; A fixing roller for fixing the toner image transferred to the medium onto the recording medium, a timing roller for supplying the recording medium to the transfer roller, and each of the timing roller and the transfer roller being the same. The length in the transport direction of the recording medium existing between the first position where the timing roller contacts the recording medium and the second position where the transfer roller contacts the recording medium while contacting the recording medium has the first position and the second position. Transport control means for transporting the recording medium to the timing roller at a feed speed higher than the transfer speed at which the transfer roller transports the recording medium in order to transport the recording medium in a first state longer than the first distance between Conveying speed control means for controlling a fixing speed at which the roller conveys the recording medium, wherein the conveying speed control means controls the fixing speed of the recording medium existing between the second position and the third position where the fixing roller is in contact with the recording medium. In order to convey the recording medium in the second state where the length in the conveyance direction is longer than the second distance between the second position and the third position, the first speed is higher than the transfer speed and the second speed is lower than the transfer speed. Speed Run the switching control to switch to, the timing roller preparatory period before release time of longer contact with the recording medium is set to a first speed fixing speed without executing the switching control.

  According to this aspect, since the recording medium is conveyed in the first state and the second state, the force for urging the rotating shaft of the transfer roller is reduced, and the release point at which the timing roller transitions to a state in which the timing roller does not contact the recording medium. , The fixing speed is set to the first speed higher than the transfer speed during the preparation period. Therefore, the force for urging the rotation axis of the transfer roller at the time of release is not too small, so that the rate at which the toner image formed on the image carrier is transferred to the recording medium at the time of release can be prevented from abruptly changing. . As a result, it is possible to provide an image forming apparatus with reduced image quality deterioration.

  Preferably, when the basis weight of the recording medium is equal to or less than the first threshold value, the transport speed control unit performs the switching control during the preparation period without setting the fixing speed to the first speed.

  According to this aspect, when the grammage of the recording medium is equal to or less than the first threshold value, the switching control is executed during the preparation period because the force for urging the rotation shaft of the transfer roller does not become too small. For this reason, it is possible to reduce the deterioration of the image quality of the toner image transferred to the recording medium whose basis weight is equal to or less than the first threshold value.

  Preferably, when the length of the recording medium in the transport direction is equal to or less than the second threshold value, the transport speed control unit executes the switching control without setting the fixing speed to the first speed during the preparation period.

  When the recording medium has a length such that the timing roller, the transfer roller, and the fixing roller do not come into contact with the recording medium, the force for urging the rotation axis of the transfer roller does not become too small. For this reason, it is possible to reduce the deterioration of the image quality of the toner image transferred to the recording medium whose length is equal to or less than the second threshold value.

  Preferably, a loop detection that detects that the amount of deflection of the recording medium existing between the timing roller and the transfer roller is equal to or greater than a predetermined upper limit or equal to or less than a predetermined lower limit. Means for switching the fixing speed to the first speed in response to the loop detecting means detecting that the amount of deflection of the recording medium has exceeded the upper limit, and The fixing speed is switched to the second speed in response to detecting that the deflection amount of the recording medium has become equal to or less than the lower limit value.

  According to this aspect, the fixing speed is switched to the first speed when the deflection amount of the recording medium is equal to or higher than the upper limit value, and the fixing speed is changed to the first speed when the deflection amount of the recording medium is equal to or lower than the lower limit value. It can be switched to two speeds. Therefore, the recording medium can be transported in the second state.

  Preferably, the conveyance speed control means executes the switching control while the transfer roller and the fixing roller are in contact with the same recording medium after the release point.

  According to this aspect, since the switching control is performed while the transfer roller and the fixing roller are in contact with the same recording medium after the release point, the deterioration of the image quality of the toner image transferred to the recording medium after the release point is reduced. can do.

  According to another aspect of the present invention, an image forming method is an image forming method executed by an image forming apparatus, the image forming apparatus including a driving roller, and forming a toner image on an image carrier. Means, a transfer roller which rotates following the drive roller and transfers the toner image formed on the image carrier to a recording medium, and a rotation axis of the transfer roller is arranged so that a distance from the rotation axis of the drive roller can be changed. In the surface having the normal axis with the rotation axis of the transfer roller, the drive roller is arranged on the surface including the rotation axis of the transfer roller with the line connecting the rotation axis of the drive roller and the rotation axis of the transfer roller as the normal line. Urging means for urging the rotation axis of the transfer roller to the side where the transfer roller is disposed; a fixing roller for fixing the toner image transferred to the recording medium to the recording medium; and a timing roller for supplying the recording medium to the transfer roller. And a recording medium existing between a first position where the timing roller contacts the recording medium and a second position where the transfer roller contacts the recording medium while the timing roller and the transfer roller each contact the same recording medium. Paper feed speed higher than the transfer speed at which the transfer roller conveys the recording medium in order to convey the recording medium in the first state in which the length in the conveyance direction is longer than the first distance between the first position and the second position A transport control step of transporting the recording medium to the timing roller, and a transport speed control step of controlling a fixing speed at which the fixing roller transports the recording medium. In the second state, the length of the recording medium existing between the third position in contact with the medium and the length in the transport direction is longer than the second distance between the second position and the third position. Executing switching control for sequentially switching the fixing speed between a first speed faster than the transfer speed and a second speed slower than the transfer speed in order to convey the image, and a preparation period before the release point when the timing roller does not contact the recording medium. Setting the fixing speed to the first speed without executing the switching control.

  According to this aspect, it is possible to provide an image forming method in which deterioration of image quality is reduced.

  According to still another aspect of the present invention, the image forming program is an image forming program executed by a computer that controls the image forming apparatus, wherein the image forming apparatus includes a driving roller, and includes a toner image on an image carrier. An image forming means for forming a toner image, a transfer roller which rotates following the drive roller and transfers the toner image formed on the image carrier to a recording medium, and a rotation axis of the transfer roller is a distance from the rotation axis of the drive roller. Are arranged so as to be variable, and on the surface having the rotation axis of the transfer roller as the normal line, the line connecting the rotation axis of the drive roller and the transfer roller with the rotation axis as the normal line is provided on the surface including the rotation axis of the transfer roller. An urging means for urging the rotation axis of the transfer roller to the side where the drive roller is disposed, a fixing roller for fixing the toner image transferred to the recording medium to the recording medium, A timing roller that supplies a recording medium to the roller; a first position where the timing roller contacts the recording medium while the timing roller and the transfer roller each contact the same recording medium; and a second position where the transfer roller contacts the recording medium. The transfer roller transports the recording medium to transport the recording medium in a first state in which the length in the transport direction of the recording medium existing between the first position and the second position is longer than a first distance between the first position and the second position. Causing the computer to execute a transport control step of transporting the recording medium to the timing roller at a feed speed higher than the transfer speed, and a transport speed control step of controlling a fixing speed at which the fixing roller transports the recording medium, and controlling the transport speed. The step is a step of determining a length of the recording medium, which is present between the second position and the third position where the fixing roller is in contact with the recording medium, in the transport direction Switches the fixing speed between a first speed higher than the transfer speed and a second speed lower than the transfer speed in order to convey the recording medium in the second state longer than the second distance between the second position and the third position. Executing the switching control; and setting the fixing speed to the first speed without executing the switching control during a preparation period before the release point when the timing roller does not contact the recording medium.

  According to this aspect, it is possible to provide an image forming program in which deterioration of image quality is reduced.

FIG. 2 is a perspective view illustrating an appearance of the MFP according to the present embodiment. FIG. 2 is a block diagram illustrating an outline of a hardware configuration of the MFP according to the present embodiment. FIG. 3 is a schematic side view illustrating an internal configuration of a part of an image forming unit and a sheet feeding unit included in the MFP according to the present embodiment. FIG. 4 is a side view illustrating a configuration of a main transport path and a plurality of sub transport paths. FIG. 4 is a first diagram for explaining a change over time of a combined force FCP applied to a transfer roller. FIG. 3 is a block diagram illustrating an example of functions of a CPU included in the MFP according to the embodiment. FIG. 5 is a diagram illustrating an example of connection timing of a timing clutch and a paper feed clutch. It is a figure showing an example of a comparative example. FIG. 9 is a second diagram for describing a change over time of a combined force FCP applied to the transfer roller. 5 is a flowchart illustrating an example of the flow of an image formation control process according to the present embodiment. It is a flowchart which shows an example of the flow of a loop time determination process. It is a flowchart which shows an example of the flow of switching control determinism. 9 is a flowchart illustrating an example of a flow of a fixing speed switching theorem.

  Hereinafter, an image forming apparatus according to an embodiment of the present invention will be described with reference to the drawings. In the following description, the same components are denoted by the same reference numerals. Their names and functions are the same. Therefore, detailed description thereof will not be repeated. In the following description, an MFP will be described as an example of the image forming apparatus. In the present embodiment, paper is used as an example of the recording medium, and the medium type is plain paper, high-quality paper, or thick paper. Recording media of different media types have different basis weights.

  FIG. 1 is a perspective view illustrating an appearance of the MFP according to the present embodiment. FIG. 2 is a block diagram illustrating an outline of a hardware configuration of the MFP according to the present embodiment. Referring to FIGS. 1 and 2, MFP 100 is an example of an image forming apparatus, and includes a main circuit 110, an original reading unit 130 for reading an original, and an automatic original for conveying the original to original reading unit 130. A transport device 120, an image forming unit 140 for forming an image on a sheet based on image data, a sheet feeding unit 150 for supplying a sheet to the image forming unit 140, and an operation panel 160 as a user interface. Including.

  The automatic document feeder 120 automatically conveys a plurality of documents set on the document tray 125 one by one to a document reading position of the document reading unit 130, and an image formed on the document by the document reading unit 130 is The read document is discharged onto a document discharge tray 127. The automatic document feeder 120 includes a document detection sensor that detects a document placed on the document tray 125.

  Document reading section 130 has a rectangular reading surface for reading a document. The reading surface is formed of, for example, platen glass. The automatic document feeder 120 is connected to the main body of the MFP 100 so as to be rotatable around an axis parallel to one side of the reading surface, and can be opened and closed. A document reading unit 130 is arranged below the automatic document feeder 120, and the reading surface of the document reading unit 130 is exposed when the automatic document feeder 120 is rotated and opened. Therefore, the user can place a document on the reading surface of the document reading unit 130. The automatic document feeder 120 can change the state between an open state where the reading surface of the document reading unit 130 is exposed and a closed state where the reading surface is covered. The automatic document feeder 120 includes a state detection sensor that detects an open state of the automatic document feeder 120.

  The document reading unit 130 includes a light source that irradiates light and a photoelectric conversion element that receives light, and scans an image formed on a document placed on the reading surface. When a document is placed in the reading area, light emitted from the light source is reflected by the document, and the reflected light forms an image by the photoelectric conversion element. Upon receiving the light reflected by the document, the photoelectric conversion element generates image data by converting the received light into an electric signal. Document reading section 130 outputs image data to CPU 111 provided in main circuit 110.

  The paper feed unit 150 conveys the paper stored in first to third paper feed cassettes and manual feed cassettes described later to the image forming unit 140. The paper feeding unit 150 detects information on the paper conveyed to the image forming unit 140 as medium information. In the present embodiment, the medium information is the basis weight of the paper. The configuration and operation for detecting the medium information in the paper feeding unit 150 will be described later in detail.

  The image forming section 140 is controlled by the CPU 111 and forms an image by a known electrophotographic method. In the present embodiment, image forming section 140 forms an image on a sheet conveyed by sheet feeding section 150 under image data input from CPU 111 and image forming conditions corresponding to the medium type of the sheet. The sheet on which the image has been formed is discharged to a discharge tray 159. Image data output from CPU 111 to image forming section 140 includes image data such as print data received from outside, in addition to image data input from document reading section 130.

  The main circuit 110 includes a CPU (Central Processing Unit) 111 that controls the entire MFP 100, a communication interface (I / F) unit 112, a ROM (Read Only Memory) 113, a RAM (Random Access Memory) 114, It includes a hard disk drive (HDD) 115 as a mass storage device, a facsimile unit 116, and an external storage device 118. CPU 111 is connected to automatic document feeder 120, document reading unit 130, image forming unit 140, paper feeding unit 150, and operation panel 160, and controls the entire MFP 100.

  The ROM 113 stores a program executed by the CPU 111 or data necessary for executing the program. The RAM 114 is used as a work area when the CPU 111 executes a program. Further, the RAM 114 temporarily stores image data continuously sent from the document reading unit 130.

  Operation panel 160 is provided above MFP 100. Operation panel 160 includes a display unit 161 and an operation unit 163. The display unit 161 is, for example, a liquid crystal display (LCD), and displays an instruction menu for a user, information on acquired image data, and the like. Note that an organic EL (electroluminescence) display, for example, can be used as long as the device displays an image instead of the LCD.

  Operation unit 163 includes a touch panel 165 and a hard key unit 167. Touch panel 165 is of a capacitance type. Note that the touch panel 165 is not limited to the capacitance type, and may use other types such as a resistance film type, a surface acoustic wave type, an infrared type, and an electromagnetic induction type.

  Touch panel 165 is provided such that its detection surface is superimposed on display portion 161 on the upper or lower surface of display portion 161. Here, the size of the detection surface of touch panel 165 and the size of the display surface of display unit 161 are the same. Therefore, the coordinate system of the display surface and the coordinate system of the detection surface are the same. Touch panel 165 detects, on the detection surface, a position where the user indicates the display surface of display unit 161, and outputs the coordinates of the detected position to CPU 111. Since the coordinate system of the display surface and the coordinate system of the detection surface are the same, the coordinates output by the touch panel 165 can be replaced with the coordinates of the display surface.

  Hard key section 167 includes a plurality of hard keys. The hard key is, for example, a contact switch. Touch panel 165 detects a position specified by the user on the display surface of display unit 161. When the user operates the MFP 100, the MFP 100 is often in an upright posture. Therefore, the display surface of the display unit 161, the operation surface of the touch panel 165, and the hard key unit 167 are arranged facing upward. This is because the user can easily visually recognize the display surface of the display unit 161 and can easily specify the operation unit 163 with his / her finger.

  Communication I / F section 112 is an interface for connecting MFP 100 to a network. The communication I / F unit 112 communicates with another computer or data processing device connected to the network using a communication protocol such as TCP (Transmission Control Protocol) or FTP (File Transfer Protocol). The network to which the communication I / F unit 112 is connected is a local area network (LAN), and the connection form may be wired or wireless. The network is not limited to a LAN, but may be a wide area network (WAN), a public switched telephone network (PSTN), the Internet, or the like.

  The facsimile unit 116 is connected to a public switched telephone network (PSTN), and transmits facsimile data to the PSTN or receives facsimile data from the PSTN. The facsimile unit 116 stores the received facsimile data in the HDD 115, converts the received facsimile data into print data printable by the image forming unit 140, and outputs the print data to the image forming unit 140. Thus, the image forming unit 140 forms an image of the facsimile data received by the facsimile unit 116 on a sheet. Further, the facsimile unit 116 converts the data stored in the HDD 115 into facsimile data and transmits the data to a facsimile apparatus connected to the PSTN.

  The external storage device 118 is controlled by the CPU 111, and has a CD-ROM (Compact Disk Read Only Memory) 118A or a semiconductor memory mounted thereon. In the present embodiment, an example in which CPU 111 executes a program stored in ROM 113 will be described. However, CPU 111 controls external storage device 118 to read a program to be executed by CPU 111 from CD-ROM 118A. Alternatively, the read program may be stored in the RAM 114 and executed.

  The recording medium for storing the program to be executed by the CPU 111 is not limited to the CD-ROM 118A, but may be a flexible disk, a cassette tape, an optical disk (MO (Magnetic Optical Disc) / MD (Mini Disc) / DVD (Digital Versatile Disc)). )), A medium such as an IC card, an optical card, a mask ROM, and a semiconductor memory such as an EPROM (Erasable Programmable ROM). Further, the CPU 111 downloads the program from the computer connected to the network and stores the program in the HDD 115, or loads the program stored in the HDD 115 into the RAM 114 so that the computer connected to the network writes the program into the HDD 115. Then, the program may be executed by the CPU 111. The program referred to here includes not only a program directly executable by the CPU 111 but also a source program, a compressed program, an encrypted program, and the like.

  FIG. 3 is a schematic side view illustrating an internal configuration of a part of the image forming unit and the sheet feeding unit included in the MFP according to the present embodiment. Referring to FIG. 3, inside MFP 100, a main transport path 41 indicated by a thick dotted line is formed so as to basically extend in the up-down direction. The main transport path 41 is a path for guiding the paper transported from the paper feed unit 150 to the paper output tray 159 through the image forming unit 140. In the main transport path 41 of the present example, the lower end 30 opposite to the upper end 13 located above the image forming section 140 constitutes a carry-in port for receiving a sheet from the sheet feeding section 150. Further, the upper end portion 13 of the main transport path 41 forms a discharge port for discharging the sheet on which the image has been formed to the discharge tray 159. A paper discharge roller 48 is provided at the upper end 13 of the main transport path 41. The lower end 30 of the main transport path 41 is connected to a plurality of sub-transport paths SP1, SP2, and SP3 of the sheet feeding unit 150 described later. The paper transport direction is a direction from the paper supply unit 150 to the paper output tray 159.

  The paper feed unit 150 includes three paper feed cassettes 151, 152, 153 and a manual feed cassette 154 for storing paper as a recording medium. The three sheet cassettes 151, 152, and 153 are stacked and arranged in this order from top to bottom. A size sensor 151s is arranged corresponding to the sheet cassette 151. The size sensor 151s detects the size of the paper stored in the paper feed cassette 151. For example, the size sensor 151s detects the position of a partition provided on the sheet cassette 151. Similarly, a size sensor 152s is arranged corresponding to the sheet cassette 152, and a size sensor 153s is arranged corresponding to the sheet cassette 153. The size sensor 152s detects the size of the sheet stored in the sheet cassette 152. The size sensor 153s detects the size of a sheet stored in the sheet cassette 153.

  The manual feed cassette 154 is provided on the side wall 101 of the MFP 100 and is located below the image forming unit 140. The manual feed cassette 154 changes its state between a closed state at a position parallel to the side wall 101 and an open state at a position forming a predetermined angle with the side wall 101. Since the upper side of the manual feed cassette 154 is open in the open state, the user can replenish the sheets. A size sensor 154s is arranged corresponding to the manual feed cassette 154. The size sensor 154s detects the size of the paper stored in the manual feed cassette 154. For example, the size sensor 154s detects the position of the partition provided on the manual feed cassette 154.

  The size sensors 151s to 154s are sensors capable of detecting a tangible object. As the size sensors 151s to 154s, for example, a photoelectric sensor, a pressure sensor, or the like can be used without limitation.

  As shown by the thick dashed line in FIG. 3, in the paper feeding unit 150, the paper feeding cassette 151 extends from the upper one of the three paper feeding cassettes 151, 152, and 153 to the lower end 30 of the main transport path 41. Is formed with a sub-transport path SP1. A sub-transport path SP2 is formed to extend from the manual feed cassette 154 to the lower end 30 of the main transport path 41. Further, two transport paths 152a and 153a are formed to extend to the lower end portion 30 of the main transport path 41 from each of the sheet cassettes 152 and 153 located at the middle and lower stages among the three sheet cassettes 151, 152 and 153. . A portion having a predetermined length from the lower end 30 of the main transport path 41 of each of the two transport paths 152a and 153a is a sub-transport path SP3 shared by the two transport paths 152a and 153a.

  In MFP 100, when forming an image on a sheet, a cassette in which sheets to be subjected to image formation are stored is selected from three sheet cassettes 151, 152, and 153 and a manual cassette 154 as a target cassette. Paper to be image-formed is supplied to the main transport path 41 from one of the three sheet cassettes 151, 152, 153 and the manual cassette 154, which is selected as the target cassette, through one of the sub-transport paths SP1, SP2, SP3. You.

  The sub-conveyance path SP1 is provided with a paper supply roller 151r and a paper supply driven roller 151rA for supplying the paper stored in the paper supply cassette 151 to the main conveyance path 41. The sub-transport path SP2 is provided with a paper feed roller 154r and a driven roller 154rA for supplying the paper stored in the manual cassette 154 to the main transport path 41. The transport path 152a is provided with a paper feed roller 152r and a driven roller 152rA for supplying sheets stored in the paper feed cassette 152 to the main transport path 41 through the sub transport path SP3. Further, the transport path 153a is provided with a paper feed roller 153r and a driven roller 153rA for supplying the paper stored in the paper feed cassette 153 to the main transport path 41 through the sub transport path SP3. The relationship between the paper feed roller 151r and the paper feed driven roller 151rA includes the relationship between the paper feed roller 152r and the driven roller 152rA, the relationship between the paper feed roller 153r and the driven roller 153rA, and the relationship between the paper feed roller 154r and the driven roller 154rA. Therefore, in the following description, the relationship between the paper feed roller 151r and the paper feed driven roller 151rA will be described as an example. The paper feed roller 151r is paired with the paper feed driven roller 151rA.

  The image forming unit 140 includes image forming units 51Y, 51M, 51C, and 51K for yellow, magenta, cyan, and black, respectively. An image is formed by driving at least one of the image forming units 51Y, 51M, 51C, and 51K. When all of the image forming units 51Y, 51M, 51C, and 51K are driven, a full-color image is formed. To the image forming units 51Y, 51M, 51C, and 51K, yellow, magenta, cyan, and black printing data are input, respectively. Since the image forming units 51Y, 51M, 51C, and 51K differ only in the color of the toner to be handled, the image forming unit 51Y for forming a yellow image will be described here.

  The image forming unit 51Y includes an exposure head to which yellow print data is input, a photosensitive drum serving as an image carrier, a charging charger, a developing device, and a transfer roller 53Y. The exposure head emits a laser beam according to the received printing data (electric signal). The emitted laser light is one-dimensionally scanned by a polygon mirror provided in the exposure head, and exposes the photosensitive drum. The photoreceptor drum is uniformly charged by a charging charger after the residual toner on the surface is removed by a cleaner before receiving the exposure, and is further irradiated with an eraser lamp to eliminate the charge. The direction in which the photosensitive drum is one-dimensionally scanned is the main scanning direction. After being charged by the charging charger, the photosensitive drum is irradiated with laser light emitted by the exposure head. As a result, an electrostatic latent image is formed on the photosensitive drum. Subsequently, the toner is placed on the electrostatic latent image by the developing device to form a toner image. The toner image formed on the photosensitive drum is transferred onto the intermediate transfer belt 57 by the transfer roller 53Y.

  On the other hand, the intermediate transfer belt 57 is suspended by the driving roller 55 and the roller 55A so as not to be slackened. The rotation shaft of the drive roller 55 is supported by the main body case. The rotation shaft of the roller 55A is fixed to the main body case via an adjustment mechanism for eliminating slack of the intermediate transfer belt 57. The drive roller 55 rotates using the motor M1 as a drive source. The rotational force of the motor M1 is directly transmitted to the drive roller 55. When the drive roller 55 rotates counterclockwise in the figure, the intermediate transfer belt 57 rotates counterclockwise in the figure at a predetermined speed. With the rotation of the intermediate transfer belt 57, the roller 55A rotates counterclockwise.

  Accordingly, the image forming units 51Y, 51M, 51C, and 51K sequentially transfer the toner images onto the intermediate transfer belt 57. The intermediate transfer belt 57 carries the transferred toner image. Therefore, the intermediate transfer belt 57 is one of the image carriers. The timing at which each of the image forming units 51Y, 51M, 51C, and 51K transfers a toner image onto the intermediate transfer belt 57 is adjusted by detecting a reference mark attached to the intermediate transfer belt 57. As a result, the yellow, magenta, cyan, and black toner images are superimposed on the intermediate transfer belt 57.

  The timing roller 45, the transfer roller 47, and the fixing roller 49 are arranged in the main transport path 41 at intervals in this order from the lower end portion 30 to the upper end portion 13. The timing roller 45 is paired with the timing driven roller 45A. The timing driven roller 45 </ b> A rotates following the timing roller 45. The transfer roller 47 is paired with the drive roller 55 and rotates following the drive roller 55. The fixing roller 49 is paired with the driven roller 49A.

  The paper supplied from the paper supply unit 150 to the main transport path 41 is sent to the timing roller 45. The timing roller 45 starts rotating at the start time at which the sheet arrives at the transfer roller 47 at the timing when the toner image formed on the intermediate transfer belt 57 arrives at the transfer roller 47. Conveys paper. The paper conveyed by the timing roller 45 is pressed against the intermediate transfer belt 57 by the transfer roller 47, and by charging the transfer roller 47, yellow, magenta, cyan, superimposed on the intermediate transfer belt 57 are formed. The black toner image is transferred to the paper. The voltage applied to the transfer roller 47 is controlled by the CPU 111 so that the charge amount of the transfer roller 47 becomes a value suitable for the basis weight of the sheet.

  The sheet on which the toner image has been transferred is conveyed to the fixing roller 49 and is heated by the fixing roller 49. Thus, the toner is melted and fixed on the sheet. Thereafter, the sheet on which the image has been formed is discharged onto the discharge tray 159 from the upper end 13 of the main transport path 41 by the discharge roller 48. The temperature of the fixing roller 49 is controlled by the CPU 111 so as to be a value suitable for the basis weight of the sheet. The rotation shaft of the drive roller 55 is supported by the main body case.

  MFP 100 in the present embodiment is provided with detection device 11 having a detection area in main transport path 41. The detection device 11 includes a light projecting unit 11a and a light receiving unit 11b, and the light projecting unit 11a and the light receiving unit 11b sandwich the main conveying path 41 at a position between the lower end 30 of the main conveying path 41 and the timing roller 45. They are arranged to face each other. The light projecting unit 11a includes, for example, a light emitting element such as a light emitting diode, a driving circuit for the light emitting element, and an optical system, and emits light along its optical axis. An area in the main transport path 41 along the optical axis of the light projecting unit 11a is a detection area. The light receiving section 11b includes a light receiving element such as a photodiode, and outputs a signal corresponding to the amount of light received by the light receiving element. The detection area of the detection device 11 is an area between the light emitting unit 11a and the light receiving unit 11b.

  In the detection device 11, a predetermined amount of light is emitted from the light projecting unit 11a to the detection area. In this state, when the sheet moves across the detection area, a part of the moving sheet is irradiated with light. At this time, part of the light applied to the sheet is transmitted through the sheet, and the rest of the light is absorbed by the sheet or reflected from the sheet. The light receiving unit 11b receives the light transmitted through the sheet and outputs a signal corresponding to the amount of received light to the CPU 111.

  The CPU 111 detects that the sheet has entered the detection area based on the output signal of the light receiving unit 11b, and determines the type of medium of the sheet moving in the detection area. Specifically, the CPU 111 detects that a sheet has entered the detection area from a change in the amount of light received by the light receiving unit 11b with respect to the amount of light emitted by the light projecting unit 11a. Further, the CPU 111 calculates, as a transmittance, a ratio of a light amount of the light received by the light receiving unit 11b to a light amount of the light emitted by the light projecting unit 11a, and determines a grammage for the transmittance. Further, the basis weight for each of the plurality of medium types is obtained by performing an experiment or the like in advance, and the medium type is determined from the basis weight. Here, the medium type is plain paper, high-quality paper, and thick paper.

  The CPU 111 sets image forming conditions for the paper based on the obtained medium type. The image forming conditions of the sheet include a voltage applied to the transfer roller 47 required for appropriately transferring the toner image formed on the intermediate transfer belt 57 to the sheet, and a method of applying the toner image transferred to the sheet to the sheet. And the temperature of the fixing roller 49 required to fix the toner appropriately. The image forming condition of the sheet includes a sheet conveying speed required for transferring and fixing the toner image on the sheet. For example, the transport speed of paper having a large basis weight is set to be lower than the transport speed of paper having a small basis weight. The transport speed of the paper having a small basis weight is set faster than the transport speed of the paper having a large basis weight.

  FIG. 4 is a side view illustrating a configuration of a main transport path and a plurality of sub-transport paths. In FIG. 4, two types of hatching different from each other are added to the sub-transport routes SP1 and SP2 to facilitate understanding of the shapes of the main transport route 41 and the plurality of sub-transport routes SP1, SP2, and SP3 and their positional relationships. At the same time, two different types of dot patterns are attached to the main transport path 41 and the auxiliary transport path SP3. Further, in the main transport path 41 and the sub-transport path SP1, an example of a path through which a sheet transported from the paper feed cassette 151 passes is indicated by a dotted line.

  The detection device 11 is arranged so as to have a detection area DA in the main transport path 41. The detection area DA of the detection device 11 intersects with the traveling direction of the sheet and extends in a direction intersecting the sheet moving on the main transport path 41. The detection area DA is an area between the light emitting unit 11a and the light receiving unit 11b. The direction in which the detection area DA extends is parallel to a line connecting the center of the light projecting unit 11a and the center of the light receiving unit 11b.

  Each of the timing roller 45 and the paper feed rollers 151r, 152r, 153r, and 154r uses the motor M1 as a drive source. The rotation shafts of the timing roller 45 and the paper feed rollers 151r, 152r, 153r, 154r are supported by the main body case. A timing clutch 45B is arranged between the motor M1 and the timing roller 45. The rotational force of the motor M1 is transmitted to the timing roller 45 via the timing clutch 45B. The timing clutch 45B is controlled by the CPU 111 and changes its state between a disconnected state and a connected state. When the timing clutch 45B is in the connected state, the rotational force of the motor M1 is transmitted to the timing roller 45, and when the timing clutch 45B is in the disconnected state, the rotational force of the motor M1 is not transmitted to the timing roller 45.

  A paper feed clutch 151rB is arranged between the motor M1 and the paper feed roller 151r. The rotational force of the motor M1 is transmitted to the paper feed roller 151r via the paper feed clutch 151rB. The paper feed clutch 151rB is controlled by the CPU 111, and changes the state between a disconnected state and a connected state. When the paper supply clutch 151rB is in the connected state, the rotational force of the motor M1 is transmitted to the paper supply roller 151r, and when the paper supply clutch 151rB is in the disconnected state, the rotational force of the motor M1 is not transmitted to the paper supply roller 151r. Similarly, a paper feed clutch 152rB is arranged between the motor M1 and the paper feed roller 152r, and a paper feed clutch 153rB is arranged between the motor M1 and the paper feed roller 153r. A paper feed clutch 154rB is arranged therebetween.

  The fixing roller 49 rotates using a motor M2 provided separately from the motor M1 as a driving source. The rotational force of the motor M2 is directly transmitted to the fixing roller 49. Therefore, it is possible to change the rotation speed of the fixing roller 49 while keeping the rotation speeds of the transfer roller 47 and the paper feed roller 151r constant.

  At least one side surface of the main transport path 41 has a predetermined curvature. A route forming member B1 that partitions the sub-transport route SP1 and the sub-transport route SP3 is provided near the connection between the lower end portion 30 of the main transport route 41 and the plurality of sub-transport routes SP1 to SP3. In addition, a path forming member B2 that separates the sub-transport path SP3 from the sub-transport path SP2 is provided. Below the path forming member B1, there is provided a path forming member B3 that separates a transfer path 152a from the sheet cassette 152 and a transfer path 153a from the sheet cassette 153.

  When the sheet is conveyed by the paper feed roller 151r and conveyed to the main conveyance path 41 through the sub conveyance path SP1, the sheet entering the main conveyance path 41 from the sub conveyance path SP1 is guided to the detection area DA. When the leading edge of the paper reaches the detection area DA, the CPU 111 detects the paper based on the conversion of the output signal of the detection device 11 and determines the medium type of the paper based on the output signal of the detection device 11. .

  Further, the sheet is transported by the sheet feed roller 151 r and contacts the timing roller 45. At this stage, the timing clutch 45B disposed on the drive path between the timing roller 45 and the motor M is disconnected, and the timing roller 45 is stopped. For this reason, the downstream end of the sheet abuts on the timing roller 45 and does not move, but the upstream portion of the sheet is pushed out by the sheet feeding roller 151r, so that the sheet bends. In the state in which the sheet is bent, the side of the leading end on the downstream side of the sheet is parallel to the timing roller 45, so that the direction of the sheet is adjusted to be a predetermined direction with respect to the timing roller 45. Therefore, when the timing clutch 45 </ b> B is engaged and the timing roller 45 rotates, the timing roller 45 can convey the sheet in a predetermined direction with respect to the timing roller 45.

  The rotation axis of the paper feed driven roller 151rA paired with the paper feed roller 151r is pivotally supported by the main body case so that the distance between the paper feed roller 151r and the rotation axis of the paper feed roller 151r varies. For example, the rotation shaft of the paper feed driven roller 151rA is supported by a paper feed roller rotating member that rotates about a paper feed fixed shaft that is supported by the main body case. The rotation of the paper feed roller rotating member causes the rotation axis of the paper feed driven roller 151rA to move relative to the rotation axis of the paper feed roller 151r. In addition, the rotation axis of the paper feed driven roller 151rA is defined by a line connecting the rotation axis of the paper feed driven roller 151r and the rotation axis of the paper feed driven roller 151rA on a plane whose normal line is the rotation axis of the paper feed driven roller 151rA. A biasing force 151rC indicated by an arrow in the drawing is applied to a side on which the sheet feeding roller 151r is disposed with respect to a plane including the rotation axis of the sheet feeding driven roller 151rA as a line. For example, a rotary spring that applies a rotational force about the paper feed fixed shaft to the paper feed roller rotating member is used. As a result, the paper feed roller 151r and the paper feed driven roller 151rA receive a force in the direction of pressing each other.

  Therefore, when a sheet exists between the sheet feeding roller 151r and the sheet feeding driven roller 151rA, a frictional force is generated between each of the sheet feeding roller 151r and the sheet feeding driven roller 151rA and the sheet. This frictional force is determined by the urging force 151rC for urging the rotation axis of the paper feed driven roller 151rA toward the paper feed roller 151r and the friction coefficient. For this reason, if the urging force 151rC is determined, the force by which the paper feed roller 151r conveys the sheet is determined. Specifically, the paper feed driven roller 151rA biases the paper feed roller 151r so that the paper feed roller 151r and the paper feed driven roller 151rA in a state where the paper feed clutch 151rB is disengaged do not rotate while holding the paper. The biasing force 151rC is set to a value equal to or larger than a predetermined lower threshold value. Specifically, the urging force 151rC for urging the paper feed driven roller 151rA from the paper feed driven roller 151r is determined by the spring coefficient of the rotary spring.

  On the other hand, when the paper feed clutch 151rB is connected, the paper conveyed by the paper feed roller 151r and the paper feed driven roller 151rA comes into contact with the timing roller 45 and bends. If the frictional force between each of the driven rollers 151rA and the sheet is too large, the timing roller 45 in a state where the timing clutch 45B is disconnected rotates. Therefore, the urging force 151rC for urging the paper feed roller 151r to urge the paper feed roller 151r so that the paper slides with the paper feed roller 151r and the paper feed driven roller 151rA before the timing roller 45 rotates. Is set to a value equal to or smaller than the upper limit threshold value.

  The rotation shaft of the timing driven roller 45 </ b> A paired with the timing roller 45 is supported by the main body case so that the distance between the rotation shaft and the rotation shaft of the timing roller 45 varies. For example, the rotation shaft of the timing driven roller 45A is supported by a timing rotation member that rotates about a fixed timing shaft that is supported by the main body case. As the timing rotation member rotates, the rotation axis of the timing driven roller 45A moves relative to the rotation axis of the timing roller 45.

  Further, the rotation shaft of the timing driven roller 45A is urged toward the timing driven roller 45A. Specifically, a line connecting the rotation axis of the timing roller 45 and the rotation axis of the timing driven roller 45A on the surface where the rotation axis of the timing driven roller 45A is normal to the rotation axis of the timing driven roller 45A is defined as the normal line. An urging force 45C indicated by an arrow in the drawing is applied to a side on which the timing roller 45 is disposed with respect to a surface including the rotation axis of the timing driven roller 45A. For example, a rotary spring that applies a rotational force about the timing fixed axis to the timing rotary member is used. Therefore, the timing roller 45 and the timing driven roller 45A receive the third biasing force in the direction of pressing each other. As a result, a frictional force is generated between the timing roller 45 and the timing driven roller 45A, and the rotation of the timing roller 45 is restricted with the timing clutch 45B disengaged. This makes it possible to minimize the amount of paper falling while the timing roller 45 is being conveyed while the timing clutch 45B is disconnected.

  The urging force 45C by which the timing driven roller 45A urges the timing roller 45 is applied when the sheet conveyed by the sheet feeding roller 151r comes into contact with the timing roller 45 and the sheet enters between the timing roller 45 and the timing driven roller 45A. Is set so that the timing roller 45 does not rotate. Specifically, the urging force 45C is determined by the spring coefficient of the rotary spring.

  The transfer roller 47 is an elastic roller formed by adding an ion conductive substance to NBR rubber and foaming the same. Since the electric resistance changes under the influence of temperature and humidity, the temperature and humidity are measured by a temperature and humidity sensor provided in the main body housing. Then, the voltage applied to the transfer roller 47 is adjusted. The rotation axis of the transfer roller 47 is supported by the main body case so that the distance between the rotation axis of the transfer roller 47 and the rotation axis of the drive roller 55 varies. For example, the rotation shaft of the transfer roller 47 is supported by a transfer rotation member that rotates about a transfer fixed shaft that is supported by the main body case. The rotation of the transfer rotation member causes the rotation axis of the transfer roller 47 to move relative to the rotation axis of the drive roller 55. The transfer roller 47 is separated from the intermediate transfer belt 57 when printing is not performed. Thereby, the creep deformation of the transfer roller 47 is prevented.

  Further, a first urging force F1 indicated by an arrow in the drawing is applied to the rotation axis of the transfer roller 47 on the drive roller 55 side. Specifically, the transfer roller 47 has a rotation axis whose normal line is a line connecting the rotation axis of the drive roller 55 and the rotation axis of the transfer roller 47 on a plane whose normal line is the rotation axis of the transfer roller 47. 47 is urged toward the side on which the drive roller 55 is disposed with respect to the plane including the rotation axis. For example, a rotary spring that applies a rotational force about the transfer fixed shaft to the transfer rotary member is used. Therefore, the driving roller 55 and the transfer roller 47 receive the first urging force F1 in the direction of pressing each other. An intermediate transfer belt 57 exists between the driving roller 55 and the transfer roller 47, and the toner image carried by the intermediate transfer belt 57 is formed in a state where the paper has entered between the intermediate transfer belt 57 and the transfer roller 47. Transferred to paper. Therefore, the magnitude of the first urging force F1 for urging the transfer roller 47 toward the drive roller 55 is set to an appropriate value to affect the image quality of the toner image formed on the sheet.

  In order to improve the image quality of the toner image on the sheet by the transfer roller 47, the first state in which a part of the sheet existing between the timing roller 45 and the transfer roller 47 is bent, and the transfer between the transfer roller 47 and the fixing roller 49 The sheet is conveyed in a second state in which a part of the sheet present is bent. The first state is the state of the sheet existing between the first position where the timing roller 45 contacts the sheet and the second position where the transfer roller 47 contacts the sheet while the timing roller 45 and the transfer roller 47 each contact the same sheet. In this state, the length in the transport direction is longer than the first distance between the first position and the second position. The second state is the state of the sheet existing between the second position where the transfer roller 47 contacts the sheet while the transfer roller 47 and the fixing roller 49 contact the same sheet and the third position where the fixing roller 49 contacts the sheet. In this state, the length in the transport direction is longer than the second distance between the second position and the third position.

  Specifically, the sheet feeding speed, the transfer speed, and the fixing speed are made different. The paper feeding speed is a speed at which the timing roller 45 conveys the paper. The transfer speed is a speed at which the transfer roller 47 conveys the sheet. The fixing speed is a speed at which the fixing roller 49 conveys the sheet. The transfer speed is the same as the speed at which the toner image is formed on the intermediate transfer belt 57. More specifically, the sheet feeding speed is set higher than the transfer speed. Accordingly, the length of the recording medium existing between the timing roller 45 and the transfer roller 47 increases with time. The fixing speed is sequentially switched between a first speed higher than the transfer speed and a second speed lower than the transfer speed.

  The loop detection sensor 12 is disposed between the transfer roller 47 and the fixing roller 49 on the transport path. The loop detection sensor 12 is attached so as to be rotatable about a rotation shaft 12A, and is urged in a counterclockwise direction in the figure by a rotation spring. The loop detection sensor 12 detects a lower limit position indicated by a solid line in the drawing and an upper limit position indicated by a dotted line in the drawing. Specifically, the loop detection sensor 12 outputs a lower limit signal while rotating counterclockwise from the lower limit position, and the loop detection sensor 12 outputs an upper limit signal while rotating clockwise from the upper limit position. I do. Note that, instead of the loop detection sensor 12, a distance measuring sensor that measures a distance may be used to detect the position of the sheet in the transport path and detect the amount of bending of the sheet.

  The fixing speed of the sheet conveyed by the transfer roller 47 is set to the second speed when the leading end reaches the fixing roller 49. Thereafter, since the length of the portion of the sheet existing between the transfer roller 47 and the fixing roller 49 increases with the passage of time, the sheet pushes the loop detection sensor 12 and the loop detection sensor 12 rotates clockwise in the figure. I do. Then, when the loop detection sensor 12 reaches the lower limit position, the loop detection sensor 12 outputs a lower limit signal. Further, when the length of the portion of the sheet existing between the transfer roller 47 and the fixing roller 49 increases with time, the sheet pushes the loop detection sensor 12 and the loop detection sensor 12 reaches the upper limit position. , The loop detection sensor 12 outputs an upper limit signal. At this stage, it is detected that the sheet bends and the amount of deflection reaches the upper limit. When it is detected that the amount of deflection of the sheet has reached the upper limit, the fixing speed is set to the second speed. Thereafter, since the length of the portion of the sheet existing between the transfer roller 47 and the fixing roller 49 decreases over time, the loop detection sensor 12 rotates counterclockwise in the figure. Then, when the loop detection sensor 12 outputs the lower limit, it is detected that the amount of bending of the sheet has reached the lower limit. Thereafter, the fixing speed is set to the second speed.

By bending a part of the sheet existing between the timing roller 45 and the transfer roller 47, a second urging force F2 indicated by an arrow in the drawing acts on the transfer roller 47. Further, by bending a part of the sheet existing between the transfer roller 47 and the fixing roller 49, a third urging force F3 indicated by an arrow in the drawing acts on the transfer roller 47. Therefore, the combined force FCP that urges the rotation axis of the transfer roller 47 toward the drive roller 55 includes a first urging force F1 that is a force urged by the rotation spring of the rotation axis of the transfer roller 47, and a transfer roller 47. Is the combined force of the second urging force F2, which is the force of the rotation shaft of the roller on the timing roller 45 side, and the third urging force F3, which is the force on the fixing roller 49 side, and is expressed by the following equation (1). Is shown.
FCP = F1-F2-F3 (1)
Here, the combined force FCP applied to the transfer roller 47 will be described.

  When the resultant force FCP is positive, the transfer roller 47 contacts the intermediate transfer belt 57. Therefore, when the paper enters between the intermediate transfer belt 57 and the transfer roller 47, the paper is pressed against the intermediate transfer belt 57 and comes into contact therewith. On the other hand, when the resultant force FCP is zero or negative, the transfer roller 47 separates from the intermediate transfer belt 57. For this reason, when the paper enters between the intermediate transfer belt 57 and the transfer roller 47, the paper is pressed against the transfer roller 47, and the paper and the intermediate transfer belt 57 are separated without contact. In this state, all the toner images formed on the intermediate transfer belt 57 are not transferred to the sheet, but a part is transferred. Here, the ratio of the toner transferred to the paper among the toners forming the toner image formed on the intermediate transfer belt 57 is defined as a transfer rate. If the intermediate transfer belt 57 is in contact with the sheet, the transfer rate is substantially constant. However, if the intermediate transfer belt 57 is separated from the sheet, the transfer rate decreases as the distance increases. If the change in the transfer rate is gentle, the deterioration of the image quality is not visually noticeable, but if the change in the transfer rate is sharp, the deterioration of the image quality is visually noticeable. For example, when the intermediate transfer belt 57 and the paper change from a separated state to a contact state, the transfer rate sharply increases, so that the image quality deteriorates. Specifically, when the transfer rate increases, the amount of toner increases, so that streak-like noise appears. Hereinafter, a value at which the resultant force FCP becomes 0 or less is referred to as a threshold value TF.

  It is conceivable to set the first urging force F1 to a large value so that the resultant force PCP does not become 0 or less. However, the greater the force with which the transfer roller 47 is urged against the intermediate transfer belt 57, the more easily the intermediate transfer belt 57 as an image carrier is deteriorated. For this reason, the upper limit value of the first urging force F1 is set.

  FIG. 5 is a first diagram for explaining a change over time of the resultant force FCP applied to the transfer roller. Referring to FIG. 5, the output signal of the loop detection sensor 12, the fixing speed, the sheet feeding speed, the transfer speed, the second urging force F2, the third urging force F3, and the combined force FCP applied to the transfer roller are shown in order from the top. ing. The first biasing force F1 is not shown in the drawing because it is constant regardless of the passage of time. Referring to FIG. 5, the fixing speed is set to the second speed when the upper limit value is detected by loop detection sensor 12, and the fixing speed is set to the first speed when the lower limit value is detected by loop detection sensor 12. Set to speed. The transfer speed is constant at V1. Therefore, the third urging force F3 increases while the fixing speed is set to the second speed, and decreases while the fixing speed is set to the first speed.

  The paper feeding speed is constant at V2 and is higher than the transfer speed V1. For this reason, the third urging force F3 increases with the passage of time. Then, at the release time t1 at which the rear end of the sheet passes through the timing roller 45, the bending of a part of the sheet existing between the timing roller 45 and the transfer roller 47 is eliminated, so that the third urging force F3 suddenly increases by ΔF. And becomes zero.

  The resultant force FCP is represented by the above equation (1). For this reason, the value of the resultant force FCP immediately before the release time t1 is smaller than the threshold value TF, but immediately after the release time t1, the value increases by the change amount ΔF and becomes larger than the threshold value TF. As described above, the state where the resultant force FCP is smaller than the threshold value TF suddenly changes to a state where it is larger than the threshold value TF. Since the value of the resultant force FCP affects the image quality of the toner image formed on the sheet, it is preferable that the value be larger than the threshold value TF before and after the change when the value changes rapidly. MFP 100 in the present embodiment controls the speed of fixing roller 49 such that the resultant force FCP becomes greater than threshold value TF when the rear end of the sheet passes timing roller 45.

  FIG. 6 is a block diagram illustrating an example of functions of a CPU included in the MFP according to the present embodiment. The function illustrated in FIG. 6 is a function formed in CPU 111 when CPU 111 included in MFP 100 executes an image forming program stored in ROM 113, HDD 115, or CD-ROM 118A. Referring to FIG. 6, CPU 111 includes a medium type detection unit 61, a medium detection unit 63, an image formation control unit 65, a size detection unit 67, a loop detection unit 69, a transport control unit 71, and a preparation period. It includes a determination unit 73 and a transport speed control unit 75.

  The medium type detection unit 61 determines the transmittance based on the output signal output by the detection device 11, and determines the medium type of the paper based on the transmittance. The basis weight for the transmittance is determined using the conversion information. The conversion information is information that determines the correspondence between the basis weight and the transmittance. The conversion information is generated by performing an experiment or the like in advance, and is stored in the HDD 115. The medium type detection unit 61 determines the medium type for the basis weight determined based on the transmittance. It is known that the basis weight of the paper of each of the plurality of medium types falls within a predetermined range. Therefore, a correspondence table between the medium type and the range of the basis weight of the paper of the medium type may be stored in the HDD 115 in advance.

  The medium detection unit 63 detects that a sheet has entered the detection area DA based on an output signal output by the detection device 11. Specifically, the medium detection unit 63 detects that a sheet has entered the detection area DA from a change in the amount of light received by the light receiving unit 11b with respect to the amount of light emitted by the light projection unit 11a of the detection device 11. . The medium detection unit 63 outputs a detection signal to the transport control unit 71 in response to detecting that the sheet has entered the detection area DA. The detection signal indicates one of a detection state indicating that a sheet is present in the detection area DA and a non-detection state indicating that no sheet is present in the detection area DA.

  The image forming control unit 65 causes the image forming unit 140 to form an image according to an image forming instruction input by the user to the operation unit 163. The image forming unit 140 calculates the time at which the toner image reaches the transfer roller 47 based on the position of the toner image formed on the intermediate transfer belt 57, and uses the calculated arrival time as the start time determining unit of the transport control unit 71. 83.

  The size detecting section 67 detects the size of the paper stored in each of the three paper feed cassettes 151, 152, 153 and the manual feed cassette 154. Specifically, the size of the sheet is detected based on the output of each of the size sensors 151s, 152s, 153s, and 154s. The size detection unit 67 outputs the sizes of the sheets stored in the three sheet cassettes 151, 152, 153 and the manual cassette 154 to the preparation period determination unit 73 and the conveyance speed control unit 75.

  It should be noted that an association table in which the size and medium type of the paper stored in the three paper feed cassettes 151, 152, 153 and the manual feed cassette 154 are stored in the HDD 115, and the paper size and the paper size are stored. The medium type may be determined. The association table stores cassette identification information for identifying each of the three paper feed cassettes 151, 152, 153 and the manual feed cassette 154, and an associated record including the size and medium type of paper stored in the cassette. Including.

  Based on the output signal of the loop detection sensor 12, the loop detection unit 69 detects that the amount of paper deflection (the amount of loop) is the upper limit or that the amount of paper deflection is the lower limit. When the loop detection sensor 12 outputs the lower limit signal, the loop detector 69 detects that the amount of bending of the sheet has reached the lower limit, and outputs a speed increase signal to the transport speed controller 75. When the loop detection sensor 12 outputs the upper limit signal, the loop detector 69 detects that the amount of bending of the sheet has reached the upper limit, and outputs a low speed signal to the transport speed controller 75. The loop detection unit 69 outputs a speed increase signal to the conveyance speed control unit 75 from when the loop detection sensor 12 outputs the lower limit signal to when the loop detection sensor 12 outputs the upper limit signal, and the loop detection sensor 12 outputs the upper limit signal. After that, a low-speed signal is output to the transport speed control unit 75 until the lower limit signal is output.

  The transport control unit 71 controls the paper feed clutch 151rB and the timing clutch 45B. When image formation is started by the image formation control unit 65, the transport control unit 71 switches the paper feed clutch 151rB to the connected state. Accordingly, the paper feed roller 151r rotates, and the conveyance of the paper is started. Thereafter, in response to the detection signal being input from the medium detection unit 63, the conveyance control unit 71 switches the paper feed clutch 151rB to the disconnected state. Thus, the conveyance of the sheet by the sheet feeding roller 151r is stopped. Since the urging force 45C for urging the timing roller 45 is applied to the timing driven roller 45A, a frictional force is generated between the timing roller 45 and the timing driven roller 45A, and the timing roller 45B is disengaged. The rotation of 45 is regulated. As a result, the amount of the sheet that is being conveyed by the timing roller 45 while the timing clutch 45B is disconnected falls below a predetermined amount. In a state where the timing clutch 45B and the paper feed clutch 151rB are disconnected, the transport control unit 71 determines a loop time that is a loop time TR earlier than the start time t2 determined based on the timing at which the image forming unit 140 forms an image on a sheet. And the timing clutch 45B at the start time t2. Hereinafter, this will be described in more detail.

The transport control unit 71 includes a loop time determination unit 81, a start time determination unit 83, a paper feed clutch control unit 85, and a timing clutch control unit 87. The loop time determination unit 81 calculates a loop time TR. The loop time TR is a time required to rotate the paper existing between the timing roller 45 and the transfer roller 47 by a predetermined amount after rotating the paper feed roller 151r. The loop time determination unit 81 calculates the loop time TR using the following equation (2).
TR = (LR-L1 + LB1 + LM) / V2-TC + TMC (2)
L1 is a first distance between a first position where the timing roller 45 contacts the sheet and a second position where the transfer roller 47 contacts the sheet.

  LB1 is the distance from the position where the loop detection sensor 12 is arranged to the timing roller 45. TC is a disconnection time from when the detection signal is input from the medium detection unit 63 to when the paper feed clutch 151rB is disconnected. In the present embodiment, the cutting time TC is set to zero. V2 is the paper feed speed at which the paper feed roller 151r conveys the paper. LR is a length that is predetermined for each type of paper as the length in the transport direction of the bent paper existing between the paper feed roller 151r and the timing roller 45. Therefore, the loop time determination unit 81 determines the loop length LR corresponding to the basis weight determined by the medium type input from the medium detection unit 63. The strength of the paper varies depending on the grammage. When the loop length LR is large, the force with which the sheet presses the timing roller 45 increases. Therefore, by setting the loop length LR to a smaller value as the basis weight is larger, it is possible to prevent the timing roller 45 from being damaged. LM is the distance that the sheet moves in the direction opposite to the transport direction when the sheet feeding roller 151r rotates in the reverse direction due to gravity applied to the sheet while the sheet feeding clutch 151rB is disconnected. The distance LM is determined by the basis weight of the sheet. This is because the weight of the paper changes depending on the grammage. Further, the loop time determination unit 81 corrects the loop time with the maximum value TMC of the time required for the paper feed clutch 151rB to transition from the disconnected state to the connected state. Note that an average value or a median value may be used separately from the maximum value.

  The start time determination unit 83 determines the start time t2 based on the arrival time input from the image formation control unit 65. The start time t2 is a time at which the timing roller 45 starts rotating. The start time determination unit 83 sets the time before the arrival time by the time obtained by dividing the first distance L1 between the timing roller 45 and the transfer roller 47 by the sheet feeding speed V2 at which the timing roller 45 conveys the sheet. Start time t2 is determined. The start time determination unit 83 outputs the start time t2 to the paper feed clutch control unit 85 and the timing clutch control unit 87. The timing clutch control unit 87 connects the timing clutch 45B when the current time reaches the start time t2. When the output signal of the detection device 11 switches from the detection state to the non-detection state, the timing clutch control section 87 disconnects the timing clutch 45B.

  The paper feed clutch control unit 85 receives the loop time TR from the loop time determination unit 81 and the start time t2 from the start time determination unit 83. When the current time reaches a loop time before the start time t2 by a loop time TR, the paper feed clutch control unit 85 connects the paper feed clutch 151rB. When the output signal of the detection device 11 switches from the detection state to the non-detection state, the paper feed clutch control unit 85 disconnects the timing clutch 45B.

  The preparation period determination unit 73 receives the sizes of the sheets stored in the three paper feed cassettes 151, 152, 153 and the manual feed cassette 154 from the size detection unit 67, and determines the preparation period. The preparation period determination unit 73 specifies a cassette in which sheets to be subjected to image formation are stored, among the three sheet cassettes 151, 152, 153 and the manual cassette 154. For example, when the user specifies a cassette using the operation unit 163, the specified cassette is specified. The preparation period determination unit 73 specifies the size of the sheet stored in the specified cassette. The preparation period is a period before the release time. Hereinafter, a case where the sheet cassette 151 is specified will be described as an example.

  The preparation period determination unit 73 includes a release time determination unit 89. The release time determination unit 89 determines a release time t1. The release time t1 is a point in time when the timing roller 45, the transfer roller 47, and the fixing roller 49 are in contact with the sheet and the timing roller 45 is not in contact with the sheet. In other words, the release time t1 is the time when the trailing edge of the sheet passes through the timing roller 45. The release time determination unit 89 determines the time when the value obtained by dividing the length of the paper in the transport direction determined by the size of the paper input from the size detection unit 67 by the paper feeding speed V2 from the start time t2 is the release time. It is calculated as t1. The release time determination unit 89 outputs the release time t1 to the paper feed clutch control unit 85 and the timing clutch control unit 87. The preparation period is a predetermined period. The preparation period determination unit 73 sets the release time t1 as the preparation end time at which the preparation period ends, and determines the time earlier than the release time t1 by the preparation period as the preparation start time at which the preparation period starts. The preparation period determination unit 73 outputs the preparation start time and the preparation end time of the preparation period to the transport speed control unit 75.

When the first speed is V3 and the deflection amount is LR, the preparation period TB2 is expressed by the following equation (3).
TB2 = (LR-L2-TL) / V3 (3)
L2 is a second distance between the transfer roller 47 and the fixing roller 49. TL is the minimum value of the amount of deflection, and is a predetermined value. When the amount of deflection at the start time of the sheet existing between the transfer roller 47 and the fixing roller 49 is calculated, the fixing speed is set to the first speed during the preparation period, with TL as the threshold value. Alternatively, a preparation period during which the amount of deflection at the end time is equal to or greater than a predetermined threshold TL may be calculated. The amount of deflection at the start time of the sheet existing between the transfer roller 47 and the fixing roller 49 is determined by the distance that the fixing roller 49 has conveyed the sheet, the distance that the transfer roller 47 has conveyed the sheet, and the distance between the fixing roller 49 and the transfer roller. The distance from the distance 47 may be calculated.

  The transport speed control unit 75 controls the fixing speed at which the fixing roller 49 transports the sheet. The transport speed control unit 75 includes a switching control unit 91 and a maintenance control unit 93. The switching control unit 91 determines that the length of the sheet in the transport direction between the second position where the transfer roller 47 contacts the sheet and the third position where the fixing roller 49 contacts the sheet is between the second position and the third position. In order to convey the sheet in the second state longer than the second distance L2 between them, the fixing speed is sequentially switched between a first speed V3 higher than the transfer speed V1 and a second speed V4 lower than the transfer speed V1.

  Specifically, the switching control unit 91 receives a speed-up signal or a low-speed signal from the loop detection unit 69. When image formation by image forming section 140 is started, switching control section 91 sets the fixing speed to second speed V4. Specifically, the motor M2 is controlled to rotate the fixing roller 49 at a rotation speed at which the sheet is conveyed at the second speed V4. The switching control unit 91 sets the fixing speed to the first speed V3 in response to the speed increase signal being input from the loop detecting unit 69. Specifically, the motor M2 is controlled to rotate the fixing roller 49 at a rotation speed at which the sheet is transported at the first speed V3. The switching control unit 91 sets the fixing speed to the second speed V4 in response to the input of the low speed signal from the loop detecting unit 69. Specifically, the motor M2 is controlled to rotate the fixing roller 49 at a rotation speed at which the sheet is conveyed at the second speed V4. Therefore, the switching control unit 91 sets the fixing speed to the first speed V3 while the speed increasing signal is input from the loop detecting unit 69, and sets the fixing speed to the first speed V3 while the low speed signal is input from the loop detecting unit 69. Is set to the second speed V4.

  The maintenance control unit 93 maintains the fixing speed at the first speed V3 so that the switching control unit 91 does not execute the switching control during the preparation period. Specifically, if the fixing speed is set to the second speed V4 when the current time becomes the preparation start time of the preparation period, the maintenance control unit 93 sets the fixing speed to the first speed V3. . If the fixing speed is set to the first speed V3 when the current time becomes the preparation start time, the maintenance control unit 93 maintains the fixing speed at the first speed V3. The maintenance control unit 93 sets the fixing speed to the second speed V4 when the preparation end time comes, and causes the switching control unit 91 to execute the switching control.

  If the basis weight of the sheet specified by the medium type input from the medium type detection unit 61 is equal to or less than the first threshold value TH1, the transport speed control unit 75 disables the function of the maintenance control unit 93 during the preparation period. , The function of the switching control unit 91 is enabled. The transport speed control unit 75 causes the switching control unit 91 to execute the switching control without maintaining the fixing speed at the first speed V3 during the preparation period. The paper having a basis weight equal to or less than the first threshold value TH1 has a weaker stiffness than the paper having a larger basis weight. Therefore, the second urging force F2 and the third urging force F3 are small, and the composite force TCP is threshold. This is because there is no case where the value is less than the value TF.

  If the length of the sheet in the sheet conveyance direction specified by the sheet size input from the size detection unit 67 is smaller than the second threshold value TH2, the conveyance speed control unit 75 sets the maintenance control unit 93 during the preparation period. Are made invalid, and the switching control unit 91 is made effective. The second threshold value TH2 is a value smaller than the minimum value of the length of the sheet when the timing roller 45, the transfer roller 47, and the fixing roller 49 are in contact with the same sheet at the same time. Therefore, the second threshold value TH2 is the sum of the first distance L1 between the timing roller 45 and the transfer roller 47 and the second distance L2 between the transfer roller 47 and the fixing roller 49. If the length of the sheet in the transport direction is smaller than the second threshold value, the sheet does not come into contact with the timing roller 45, the transfer roller 47, and the fixing roller 49 at the same time, so the second urging force F2 and the third urging force F3 act simultaneously. This is because there is no case where the resultant force TCP does not fall below the threshold value TF.

  FIG. 7 is a diagram illustrating an example of the connection timing of the timing clutch and the paper feed clutch. Referring to FIG. 7, the horizontal axis indicates the flow of time, and from the top, the output signal of detection device 11, the connection state of timing clutch 45B, and the connection state of paper feed clutch 152rB. The output signal of the detection device 11 indicates a detection state in which the sheet is detected and a non-detection state in which the sheet is not detected.

  When image formation by the image forming unit 140 is started, the paper feed clutch 151rB is connected. When the state of the output signal of the detection device 11 switches from the non-detection state to the detection state, the paper feed clutch 151rB is disconnected. Next, the paper feed clutch 151rB is connected at a loop time before the disclosure time by a loop time TR. Then, at the start time when the loop time TR has elapsed, the timing clutch 45B is connected. A loop time TR is secured between the time when the paper feed clutch 151rB is connected and the time when the timing clutch 45B is connected. For this reason, the leading end of the sheet conveyed by the sheet feeding roller 151 r abuts on the timing roller 45 and is bent. When the sheet bends, the leading end of the sheet becomes parallel to the rotation axis of the timing roller 45, so that the sheet conveyance direction is aligned. Then, when the state of the output signal of the detection device 11 switches from the detection state to the non-detection state, the paper feed clutch 151rB and the timing clutch 45B are disconnected.

  FIG. 8 is a diagram illustrating an example of a comparative example. With reference to FIG. 8, the horizontal axis indicates the flow of time, and from the top, the output signal of the detection device 11, the connection state of the timing clutch, and the connection state of the paper feed clutch. The state of the output signal of the detection device 11 indicates a state in which the sheet is detected and the sheet is detected, and a state in which the sheet is not detected.

  When image formation by the image forming unit 140 is started, the paper feed clutch 151rB is connected. After a lapse of a predetermined time since the state of the output signal of the detection device 11 switches from the non-detection state to the detection state, the paper feed clutch 151rB is disconnected. The predetermined time is a time until the leading end of the sheet comes into contact with the timing roller 45 and bends. The predetermined time is predetermined as a time that elapses after the state of the output signal of the detection device 11 switches from the non-detection state to the detection state. Next, at the disclosure time, the paper feed clutch 151rB and the timing clutch 45B are connected. Then, when the state of the output signal of the detection device 11 switches from the detection state to the non-detection state, the paper feed clutch 151rB and the timing clutch 45B are disconnected.

  7 and FIG. 8, the timing of the loop time during which the sheet is bent between the sheet feeding roller 151 r and the timing roller 45 is different. In FIG. 7, this is after the paper feed clutch 151rB is connected, whereas in FIG. 8, it is before the paper feed clutch 151rB is disconnected. In the comparative example in FIG. 8, the paper feed clutch 151rB is disconnected in a state where the paper is bent. In this case, when the basis weight of the paper is large and the strain is strong, the paper feed roller 151r may rotate in the reverse direction. In this case, the leading end of the paper may not be parallel to the rotation axis of the timing roller 45. is there. On the other hand, as shown in FIG. 7, when the paper feed clutch 151rB is connected in a state where the paper is bent, the paper feed clutch 151rB does not rotate in the reverse direction, and the leading end of the paper becomes the timing roller. The timing roller 45 can be rotated while maintaining a state parallel to the rotation axis of the timing roller 45.

  FIG. 9 is a second diagram for describing a temporal change in the resultant force FCP applied to the transfer roller. Referring to FIG. 9, the output signal of the loop detection sensor 12, the fixing speed, the paper feeding speed, the transfer speed, the second urging force F2, the third urging force F3, and the combined force FCP applied to the transfer roller are shown in order from the top. ing. The first biasing force F1 is not shown in the drawing because it is constant regardless of the passage of time. Referring to FIG. 9, the second urging force F2 and the resultant force FCP are different from those shown in FIG. In the preparation period before the release time t1, the fixing speed is set to the first speed V3. The transfer speed is constant at V1. Therefore, the third urging force F3 decreases without increasing in the preparation period.

  The third urging force F3 increases with the passage of time during the preparation period, rapidly decreases at the release time t1, and becomes zero. The resultant force FCP is represented by the above equation (1). Immediately before the release time t1, the value of the resultant force FCP is larger than the threshold value TF, and immediately after the release time t1, the value increases by the change amount ΔF but is larger than the threshold value TF. As described above, the state where the composite force FCP is greater than the threshold value TF before and after the trailing edge of the sheet passes through the timing roller 45 is maintained, so that the change in the transfer rate is reduced. Therefore, the effect on the image quality of the toner image transferred to the paper can be reduced.

  FIG. 10 is a flowchart illustrating an example of the flow of the image formation control process according to the present embodiment. The image forming control process is a process executed by CPU 111 of MFP 100 as CPU 111 executes an image forming control program stored in ROM 113, HDD 115, or CD-ROM 118A. Referring to FIG. 10, CPU 111 provided in MFP 100 starts image formation (step S01), and advances the process to step S02. At this stage, the motor M1 is driven, and the drive roller 55 rotates.

  In step S02, the size of a sheet on which an image is to be formed is detected. From among the three paper feed cassettes 151, 152, 153 and the manual feed cassette 154, the cassette storing the paper to be subjected to image formation is specified. For example, when the user specifies a cassette using the operation unit 163, the specified cassette is specified. Here, an example in which the sheet cassette 151 is specified will be described.

  In the next step S03, the paper feed clutch 151rB is connected, and the process proceeds to step S04. In step S04, the fixing speed is set to the second speed, and the process proceeds to step S05. In step S05, the fixing roller is driven, and the process proceeds to step S06. Thus, the fixing roller 49 starts rotating at the number of rotations for conveying the sheet at the second speed.

  In step S06, it is determined whether a sheet is detected in detection area DA. Specifically, when the output signal of the detection device 11 indicates the detection state, it is detected that the sheet has entered the detection area DA. The process waits until a sheet is detected (NO in step S06). If a sheet is detected (YES in step S06), the process proceeds to step S07. In step S07, the paper feed clutch 151rB is disconnected, and the process proceeds to step S08.

  In the next step S08, the medium type is determined, and the process proceeds to step S09. The medium type of the paper is determined based on the output signal of the detection device 11. In step S09, a loop time determination process is performed, and the process proceeds to step S10. The loop time determination process is a process for determining a loop time TR, a start time, and a loop time. In the next step S10, a switching control determination process is executed, and the process proceeds to step S11. Although details of the switching control determination processing will be described later, this processing determines whether or not to execute the switching control during the preparation period.

  In the next step S11, it is determined whether or not the current time is a loop time. The process waits until the current time reaches the loop time (NO in step S11). If the current time is the loop time (YES in step S11), the process proceeds to step S12.

  In step S12, the paper feed clutch 151rB is connected, and the process proceeds to step S13. Thereby, the conveyance of the sheet by the sheet feeding roller 151r is started. In the next step S13, it is determined whether or not the current time is the start time. The process waits until the current time reaches the start time (NO in step S13). If the current time is the start time (YES in step S13), the process proceeds to step S14. In step S14, the timing clutch 45B is connected, and the process proceeds to step S15.

  In step S15, a fixing speed switching process is performed, and the process proceeds to step S16. Although details of the fixing speed switching process will be described later, this is a process of switching the fixing speed. In step S16, it is determined whether image formation has been completed. If the sheet has passed through the fixing roller 49, it is determined that the image formation has been completed. The process waits until the image formation is completed (NO in step S16). If the image formation is completed (YES in step S16), the process proceeds to step S17. In step S17, the fixing roller 49 is stopped, and the process ends. Specifically, the motor M2 is stopped.

  FIG. 11 is a flowchart illustrating an example of the flow of the loop time determination process. The loop time determination process is a process executed in step S09 of FIG. Referring to FIG. 11, CPU 111 determines loop time TR (step S21). The loop time TR is calculated using the above equation (2). The loop time TR is determined using the loop length LR corresponding to the grammage determined by the medium type of the paper determined in step S08. The strength of the paper varies depending on the grammage. When the loop length LR is large, the force with which the sheet presses the timing roller 45 increases. Therefore, by setting the loop length LR to a smaller value as the basis weight is larger, it is possible to prevent the timing roller 45 from being damaged.

  In the next step S22, a start time t2 is determined. The image forming unit 140 determines the time at which the toner image reaches the transfer roller 47 as the arrival time based on the position of the toner image formed on the intermediate transfer belt 57. The start time t2 is determined to be a time earlier than the arrival time determined by the image forming unit 140 and obtained by dividing the first distance L1 between the timing roller 45 and the transfer roller 47 by the sheet feeding speed V2. .

  In the next step S23, the loop time is determined, and the process returns to the image forming control process. A time earlier than the start time t2 determined in step S22 by the loop time TR determined in step S21 is determined as a loop time.

  FIG. 12 is a flowchart illustrating an example of the flow of the switching control determinant. The switching control determination process is a process executed in step S10 of FIG. Referring to FIG. 12, CPU 111 determines release time t1 (step S31), and advances the process to step S32. The release time t1 is a time at which the trailing edge of the sheet passes through the timing roller 45. The release time t1 is determined as the time at which the time obtained by dividing the length of the paper in the transport direction determined by the paper size detected in step S02 by the paper feeding speed V2 from the start time has elapsed.

  In the next step S32, a preparation period is determined, and the process proceeds to step S33. A predetermined period is determined as the preparation period as the preparation period. In step S33, the preparation start time is determined, and the process proceeds to step S34. The time before the release time determined in step S31 by the preparation period is determined as the preparation start time.

  In step S34, it is determined whether the grammage is equal to or less than a first threshold value TH1. If the basis weight determined by the medium type of the paper determined in step S08 is equal to or less than the first threshold value TH1, the process proceeds to step S35; otherwise, the process proceeds to step S36. In step S35, the switching control flag is set to ON, and the process returns to the image forming control process. In step S36, it is determined whether or not the paper size is equal to or smaller than second threshold value TH2. If the length of the sheet in the transport direction determined by the sheet size detected in step S02 is equal to or smaller than the second threshold TH2, the process proceeds to step S35; otherwise, the process proceeds to step S37. In step S37, the switching control flag is set to OFF, and the process returns to the image forming control process.

  FIG. 13 is a flowchart illustrating an example of the flow of the fixing speed switching theorem. The fixing speed switching theorem is a process executed in step S15 of FIG. Referring to FIG. 13, CPU 111 determines whether or not the loop amount has reached the upper limit (step S41). If the loop detection sensor 12 detects that the amount of sheet deflection has reached the upper limit, it is determined that the loop amount has exceeded the upper limit. If the loop amount is at the upper limit, the process proceeds to step S42; otherwise, the process skips step S42 and proceeds to step S43. In step S42, the fixing speed is set to the first speed, and the process proceeds to step S43.

  In step S43, it is determined whether the loop amount has reached the lower limit. When the loop detection sensor 12 detects that the amount of sheet deflection has reached the lower limit, it is determined that the loop amount has reached the lower limit. If the loop amount is the lower limit, the process proceeds to step S44; otherwise, step S44 is skipped and the process proceeds to step S45. In step S44, the fixing speed is set to the second speed, and the process proceeds to step S45.

  In step S45, it is determined whether the current time is after the preparation start time. If the current time is after the preparation start time or after the preparation start time, the process proceeds to step S46; otherwise, the process returns to step S41. In step S46, it is determined whether the switching control flag is set to ON. If the switching control flag has been set to ON, the process proceeds to step S51; otherwise, the process proceeds to step S47.

  In step S47, the fixing speed is set to the first speed, and the process proceeds to step S48. In step S48, it is determined whether the current time is the release time t1. If the current time is the release time t1, the process proceeds to step S49; otherwise, the process returns to step S47. In step S49, the fixing speed is set to the second speed, and the process proceeds to step S50. In step S50, the control switching flag is set to ON, and the process returns to step S41.

  In step S51, it is determined whether a sheet is detected in the detection area DA. Based on the output signal of the detection device 11, it is determined whether a sheet exists in the detection area DA. If a sheet is detected in detection area DA, the process returns to step S41; otherwise, the process proceeds to step S52.

  In step S52, the timing clutch 45B is disconnected, and the process proceeds to step S53. In step S53, the paper feed clutch 151rB is disconnected, and the process returns to the image forming control process.

  The MFP 100 according to the present embodiment includes the photosensitive drums corresponding to the image forming units 51Y, 51M, 51C, and 51K. However, the cycle type developing method in which one photosensitive drum is shared by the developing units. May be adopted. In the cycle type developing method, when a full-color image is formed, each of the yellow, magenta, cyan, and black developing units sequentially forms a toner image on the photosensitive drum. Therefore, the sheet is repeatedly conveyed to the photosensitive drum so that the yellow, magenta, cyan, and black toner images are transferred to the sheet. In this case, the intermediate transfer belt 57 provided in the MFP 100 in the present embodiment is unnecessary, and the photosensitive drum serving as the image carrier serves as a driving roller for rotating the transfer roller 47.

  The embodiments disclosed this time are to be considered in all respects as illustrative and not restrictive. The scope of the present invention is defined by the terms of the claims, rather than the description above, and is intended to include any modifications within the scope and meaning equivalent to the terms of the claims.

<Appendix>
(1) The image forming apparatus according to any one of claims 1 to 6, wherein an urging force for urging the transfer roller by the urging unit is equal to or less than a predetermined upper limit. According to this aspect, deterioration of the image carrier can be reduced as much as possible.
(2) The transport speed control unit is configured to determine the transport speed of the recording medium transported by the timing roller, the paper feed speed, and a start time at which the timing roller starts transporting the recording medium. The image forming apparatus according to claim 1, wherein a release point is determined.
(3) further comprising a driven roller paired with the driving roller,
The image forming apparatus according to claim 1, wherein the image carrier is suspended between the driving roller and the driven roller.
(4) The image forming apparatus according to any one of claims 1 to 7, wherein the drive roller is the image carrier.

  100 MFP, 110 main circuit, 111 CPU, 112 communication interface (I / F) unit, 113 ROM, 114 RAM, 115 HDD, 116 facsimile unit, 118 external storage device, 118A CD-ROM, 120 automatic document feeder, 125 Document tray, 127 document discharge tray, 130 document reading unit, 140 image forming unit, 150 paper feeding unit, 151, 152, 153 paper cassette, 151s to 154s size sensor, 151r to 154r paper roller, 154 manual cassette, 159 paper output tray, 160 operation panel, 161 display unit, 163 operation unit, 165 touch panel, 167 hard key unit, 11 detection device, 12 loop detection sensor, 41 main transport path, 45 timing roller, 45A timing Moving roller, 45B timing clutch, 47 transfer roller, 48 discharge roller, 49 fixing roller, 49A driven roller, 51Y, 51M, 51C, 51K image forming unit, 55 drive roller, 57 intermediate transfer belt, 61 medium type detection unit, 63 medium detection unit, 65 image formation control unit, 67 size detection unit, 69 loop detection unit, 71 transport control unit, 73 preparation period determination unit, 75 transport speed control unit, 81 loop time determination unit, 83 start time determination unit, 85 paper feed clutch control unit, 87 timing clutch control unit, 89 release time determination unit, 91 switching control unit, 93 maintenance control unit, 151r, 152r, 153r paper feed roller, 151rA, 152rA, 153rA, 154rA paper feed driven roller, 151rB, 152rB, 153r , 154rB paper feed clutch, 151s, 152s, 153s, 154s size sensor, 151, 152, 153 paper feed cassette, 154 manual feed cassette, LR loop length, M1 motor, M2 motor, TB2 preparation period, TC disconnection time, FCP synthesis force , F1 first urging force, F2 second urging force, F3 third urging force, TR loop time, V1 transfer speed, V2 paper feed speed, t1 release time, t2 start time, ΔF change amount.

Claims (7)

An image forming unit that includes a driving roller and forms a toner image on the image carrier,
A transfer roller that rotates following the drive roller and transfers the toner image formed on the image carrier to a recording medium,
The rotation axis of the transfer roller is arranged so that the distance from the rotation axis of the drive roller can be varied, and the rotation axis of the drive roller and the rotation of the transfer roller on a plane normal to the rotation axis of the transfer roller. An urging means for urging the rotation axis of the transfer roller to the side where the drive roller is disposed with respect to a plane including the rotation axis of the transfer roller with a line connecting the axis as a normal line,
A fixing roller for fixing the toner image transferred to the recording medium to the recording medium,
A timing roller for supplying a recording medium to the transfer roller,
Conveyance of a recording medium existing between a first position where the timing roller contacts the recording medium and a second position where the transfer roller contacts the recording medium while the timing roller and the transfer roller each contact the same recording medium. In order to convey the recording medium in the first state in which the length in the direction is longer than the first distance between the first position and the second position, the feeding speed is higher than the transfer speed at which the transfer roller conveys the recording medium. Conveyance control means for conveying the recording medium to the timing roller at a speed,
A conveyance speed control unit that controls a fixing speed at which the fixing roller conveys the recording medium,
The conveyance speed control unit may be configured such that a length of the recording medium in a conveyance direction between the second position and a third position where the fixing roller is in contact with the recording medium is between the second position and the third position. Performing a switching control for sequentially switching the fixing speed between a first speed higher than the transfer speed and a second speed lower than the transfer speed in order to convey the recording medium in a second state longer than the second distance.
An image forming apparatus, wherein the fixing speed is set to the first speed without performing the switching control during a preparation period before a release point at which the timing roller is not in contact with a recording medium.   2. The transport speed control unit according to claim 1, wherein when the basis weight of the recording medium is equal to or less than a first threshold value, the switching control is performed without setting the fixing speed to the first speed in the preparation period. 3. The image forming apparatus as described in the above.   The transport speed control unit executes the switching control during the preparation period without setting the fixing speed to the first speed when the length of the recording medium in the transport direction is equal to or less than a second threshold value. Item 3. The image forming apparatus according to item 1 or 2. A loop detection unit that detects that the amount of deflection of the recording medium existing between the timing roller and the transfer roller is equal to or greater than a predetermined upper limit or equal to or less than a predetermined lower limit. In addition,
The conveyance speed control unit switches the fixing speed to the first speed in response to the loop detection unit detecting that the amount of deflection of the recording medium has become equal to or greater than the upper limit value. The image forming apparatus according to any one of claims 1 to 3, wherein the fixing speed is switched to the second speed in response to detecting that the amount of deflection of the recording medium has become equal to or less than the lower limit.   5. The image forming apparatus according to claim 1, wherein the transport speed control unit executes the switching control while the transfer roller and the fixing roller are in contact with the same recording medium after the release point. 6. . An image forming method executed by an image forming apparatus,
The image forming apparatus includes:
An image forming unit that includes a driving roller and forms a toner image on the image carrier,
A transfer roller that rotates following the drive roller and transfers the toner image formed on the image carrier to a recording medium,
The rotation axis of the transfer roller is arranged so that the distance from the rotation axis of the drive roller can be varied, and the rotation axis of the drive roller and the rotation of the transfer roller on a plane normal to the rotation axis of the transfer roller. An urging means for urging the rotation axis of the transfer roller to the side where the drive roller is disposed with respect to a plane including the rotation axis of the transfer roller with a line connecting the axis as a normal line,
A fixing roller for fixing the toner image transferred to the recording medium to the recording medium,
A timing roller that supplies a recording medium to the transfer roller,
Conveyance of a recording medium existing between a first position where the timing roller contacts the recording medium and a second position where the transfer roller contacts the recording medium while the timing roller and the transfer roller each contact the same recording medium. In order to convey the recording medium in the first state in which the length in the direction is longer than the first distance between the first position and the second position, the feeding speed is higher than the transfer speed at which the transfer roller conveys the recording medium. A conveyance control step of conveying the recording medium to the timing roller at a speed,
A transport speed control step of controlling a fixing speed at which the fixing roller transports the recording medium,
The transport speed control step may be such that the length of the recording medium existing between the second position and the third position where the fixing roller is in contact with the recording medium is between the second position and the third position. Executing a switching control for sequentially switching the fixing speed between a first speed higher than the transfer speed and a second speed lower than the transfer speed in order to convey the recording medium in a second state longer than the second distance.
Setting the fixing speed to the first speed without performing the switching control during a preparation period before a release point at which the timing roller does not come into contact with a recording medium. An image forming program executed by a computer that controls the image forming apparatus,
The image forming apparatus includes:
The image forming apparatus includes:
An image forming unit that includes a driving roller and forms a toner image on the image carrier,
A transfer roller that rotates following the drive roller and transfers the toner image formed on the image carrier to a recording medium,
The rotation axis of the transfer roller is arranged so that the distance from the rotation axis of the drive roller can be varied, and the rotation axis of the drive roller and the rotation of the transfer roller on a plane normal to the rotation axis of the transfer roller. An urging means for urging the rotation axis of the transfer roller to the side where the drive roller is disposed with respect to a plane including the rotation axis of the transfer roller with a line connecting the axis as a normal line,
A fixing roller for fixing the toner image transferred to the recording medium to the recording medium,
A timing roller that supplies a recording medium to the transfer roller,
Conveyance of a recording medium existing between a first position where the timing roller contacts the recording medium and a second position where the transfer roller contacts the recording medium while the timing roller and the transfer roller each contact the same recording medium. In order to convey the recording medium in the first state in which the length in the direction is longer than the first distance between the first position and the second position, the feeding speed is higher than the transfer speed at which the transfer roller conveys the recording medium. A conveyance control step of conveying the recording medium to the timing roller at a speed,
A conveyance speed control step of controlling a fixing speed at which the fixing roller conveys a recording medium, and
The transport speed control step may be such that the length of the recording medium existing between the second position and the third position where the fixing roller is in contact with the recording medium is between the second position and the third position. Executing a switching control for sequentially switching the fixing speed between a first speed higher than the transfer speed and a second speed lower than the transfer speed in order to convey the recording medium in a second state longer than the second distance.
Setting the fixing speed to the first speed without performing the switching control during a preparation period before a release point at which the timing roller does not come into contact with a recording medium.

Images