JP5392297B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus having a function of forming an image on a first surface and a second surface of a sheet.

A conventional electrophotographic image forming apparatus is configured as shown in FIG.
FIG. 22 is a diagram showing a schematic configuration of a tandem type color printer, which has a double-sided printing function for forming a color image on the first side and the second side of a sheet. In double-sided printing, the following operation is performed.
That is, electrostatic latent images formed on the photosensitive drums 911Y to 911K by the imaging units (IU) 901Y to 901K for Y (yellow), M (magenta), C (cyan), and K (black) colors are displayed. The developing units 912Y to 912K are visualized with toner.

The toner images visualized on the photosensitive drums 911Y to 911K are multiple transferred onto the intermediate transfer belt 921 that circulates in the direction of the arrow. The respective color toner images transferred onto the intermediate transfer belt 921 are transferred from the cassette of the feeding unit 902 and are secondarily transferred to the first surface (back surface) of the sheet S conveyed on the conveyance path 931 at the secondary transfer position 922. Transcribed.
The sheet S that has passed through the secondary transfer position 922 is conveyed to the fixing unit 903, and after the toner image is thermally fixed on the first surface of the sheet S by the fixing unit 903, the sheet S is switched back by the discharge reversing unit 904. It is sent to the transport unit 905.

The sheet S sent to the duplex conveying unit 905 is conveyed on the circulation path 951 and returned to the secondary transfer position 922 again. In parallel with this transport operation, toner image forming operations are performed by IUs 901Y to 901K, and when the sheet S passes the secondary transfer position 922, an intermediate transfer belt is formed on the second surface (front surface) of the sheet S. The toner image on 921 is transferred.
The sheet S that has passed through the secondary transfer position 922 is conveyed to the fixing unit 903, where the toner image is thermally fixed on the second surface of the sheet by the fixing unit 903, and then discharged outside the apparatus via the discharge reversing unit 904. The

  In such double-sided printing, the sheet S passes through the fixing unit 903 and then switches back, and then returns to the secondary transfer position 922 via the circulation path 951. The temperature of the sheet S remains high to some extent even when it returns to the secondary transfer position 922 due to heating when passing through 903. When the high-temperature sheet S moves to the secondary transfer position 922, the temperature of the circulation path 951 and the conveyance path 931 through which the sheet S passes increases, and the temperature of the IU 901K near the conveyance path 931 also easily increases. Further, when the sheet S passes through the secondary transfer position 922, the heat of the sheet S is transmitted to the intermediate transfer belt 921, the heat of the intermediate transfer belt 921 is transmitted to the IU 901Y to 901K, and the temperature of the IU 901Y to 901K is likely to rise. Become.

Such a temperature rise of IU 901Y to 901K becomes higher as the number of sheets to be continuously printed in the duplex printing increases.
When the temperature of the IU 901Y to 901K rises, the heat is transmitted to the developing units 912Y to 912K. In recent years, toners stored in the developing units 912Y to 912K often have a low melting point due to energy saving. Therefore, when the temperature of the developing units 912Y to 912K rises, the toner may be melted depending on the raised temperature. If the toner melts in the developing units 912Y to 912K, proper development cannot be performed. Therefore, cooling IU901Y-901K has been conventionally performed.

As a configuration for cooling the IU 901Y to 901K, for example, as shown in the figure, the heated air around the IU 901K is sucked by a fan (not shown) so as to cross the conveyance path 931 and the circulation path 951. There is a configuration in which the air is discharged out of the apparatus via the air passage 961.
In addition, as a configuration not using a fan, Patent Document 1 discloses that when double-sided printing is performed by continuously feeding a plurality of sheets, the number of prints within a certain time is limited when a predetermined number or more. A technique for preventing temperature rise in the machine is disclosed.

JP 2001-337574 A

When performing double-sided printing, in order to further improve the number of printed sheets per unit time (productivity), the interval between consecutive sheets (sheet interval) is reduced as much as possible.
When the sheet interval is reduced in this way, for example, as illustrated in FIG. 22, while the sheet S1 is being transported through the circulation path 951, the sheet S2 is being fed toward the secondary transfer position 922 while the sheet S2 is being fed. S1 is temporarily stopped (standby) in the circulation path 951, and after the sheet S2 passes the secondary transfer position 922, the conveyance of the sheet S1 to the secondary transfer position 922 is resumed. The position where the sheet S1 is waited is set in advance to a position as close as possible to the secondary transfer position 922. Specifically, the leading edge of the sheet S1 is placed on the roller pair 955 located in the most downstream in the sheet conveyance direction in the circulation path 951. It is said that the position has passed a little.

  This standby control can prevent the sheets S1 and S2 from colliding with each other on the conveyance path 931. However, since the length of the circulation path 951 is shortened due to the downsizing of the apparatus, When the sheet S1 is made to stand by, the rear end portion of the sheet S1 is applied to the air passage 961, which causes a problem that the air passage 961 is blocked. When the air passage 961 is blocked, the heated air around the IU 901K cannot be temporarily discharged to the outside.

  If the air passage 961 crosses the conveyance path 931 and the circulation path 951 as shown in the figure, the air passage 961 is blocked while the sheet S is being conveyed through one of the conveyance path 931 and the circulation path 951. When the sheet S passes through the position of the air passage 961, the air passage 961 is opened. That is, only during the sheet interval, the heated air in the peripheral space of the IU 901K flows through the air passage 961, and the temperature rise of the IUs 10Y to 10K is prevented.

The operation of temporarily stopping and resuming the conveyance of the sheet in the circulation path 951 is repeated for each sheet, and the total number of sheets that are blocked by the air path 961 increases as the number of sheets to be printed in one job increases. And the temperature of the IU is likely to rise.
In Patent Document 1, the paper interval is increased in order to limit the number of prints, and increasing the paper interval is equivalent to delaying the feeding timing of the paper S2. Thus, even if the paper S2 feeding timing is delayed, if the paper S1 is waiting in the circulation path 951, the air flow path 961 is still blocked by the paper S1, and the IU's Temperature rise cannot be suppressed.

  The present invention has been made in view of the above-described problems, and in an image forming apparatus that forms images on both sides of a sheet, the temperature of an image forming unit such as an IU is reduced as much as possible without reducing image forming productivity. An object of the present invention is to provide an image forming apparatus capable of suppressing the increase as compared with the prior art.

In order to achieve the above object, an image forming apparatus according to the present invention forms an image on a first surface of a conveyed sheet by an image forming unit, thermally fixes the image on the sheet, and passes the sheet through a circulation path. An image forming apparatus that transports the sheet back to the image forming unit, forms an image on the second surface of the sheet, heat-fixes the image on the sheet, and then discharges the sheet. Acquisition means for acquiring temperature index information, cooling means for cooling the image forming section by passing cooling air across the circulation path and leading to the image forming section, and the sheet being conveyed through the circulation path directed to the image forming section In order to take the timing to send out, the conveyance stop means for temporarily stopping and waiting in the circulation path, the cooling means different from the cooling means, and the sheet on which the image is formed on the first surface are thermally fixed, A switch that reverses the sheet on the reverse path. Comprising a back section, wherein the conveyance stop means, information the index is the one where the temperature indicates that less than the predetermined value, the sheet from the position of the sheet, the air passage across the circulation path If the leading edge is stopped at the first position located downstream of the sheet conveying direction and shows a predetermined value or more, the leading edge is stopped at the second position upstream of the first position in the sheet conveying direction and the sheet does not block the air passage. The another cooling means has an air passage different from the air passage that crosses the inversion path and communicates with the image forming section, cools the image forming section through cooling air through the air passage, and the switch If the information to be indexed indicates that the temperature is lower than a predetermined value, the back means has the sheet trailing edge positioned upstream in the sheet conveying direction from the crossing position where the other air passage crosses the reversing path. Flip the sheet at the third position to , As long as it exhibits a predetermined value or more, the sheet conveyance direction downstream and sheet than the third position, characterized in that reversing the sheet at a fourth position which does not block said further blowing path.

Further, the second position is a position that exists upstream in the sheet conveying direction from the position where the leading end of the sheet crosses.
Furthermore, the second position is a position where the front end of the sheet blocks only a part of the air passage.
The cooling means has a fan for sending cooling air to the air passage, and blows air when the sheet is stopped at the second position, and the sheet stops at the first position. It is characterized in that the air blowing is stopped when it is in operation.

Here, the cooling means stops air blowing by the fan while the air blowing path is blocked by a sheet transporting the circulation path .

In addition, when the switchback unit reverses the sheet conveyed on the reversing path at the third position, the switchback unit performs reversal after stopping at the third position for a first stop time. When reversing at a position, the reversal is performed after stopping at the fourth position for a second stop time longer than the first stop time.
Here, the reversing path is provided between the position where the sheet after image formation on the first surface is thermally fixed and the circulation path.

Further, the apparatus includes a temperature detection unit that detects the temperature, the acquisition unit acquires a detection result of the temperature detection unit as the information, and the conveyance stop unit has a temperature detected by the temperature detection unit that is greater than a predetermined value. Is lower, the sheet is stopped at the first position, and when it is equal to or greater than a predetermined value, the sheet is stopped at the second position.
Further, the acquisition unit acquires, as the information, information indicating the number of sheets in a job for performing image formation on a plurality of continuously conveyed sheets, and the conveyance stop unit includes the number of sheets. Is less than a predetermined number, it is assumed that the temperature is lower than a predetermined value, and the sheet is stopped at the first position. As a feature, the sheet is stopped at the second position.

  In addition, the acquisition unit acquires information indicating the size of the sheet to be conveyed as the information, and the conveyance stop unit has the temperature lower than a predetermined value when the sheet size is the first size. When the sheet is stopped at the first position and the second size is longer in the transport direction than the first size, the temperature is set to be equal to or higher than the predetermined value. It is stopped at 2 positions.

  The acquisition means acquires information indicating the thickness of the conveyed sheet as the information, and the conveyance stop means determines that the temperature is less than a predetermined value when the thickness of the sheet is the first thickness. The sheet is stopped at the first position, and when the second thickness is greater than the first thickness, the sheet is It is characterized by being stopped at a position.

  And a conveyance roller pair that is provided in the circulation path and conveys the sheet, and a conveyance motor that rotationally drives the conveyance roller pair, and the acquisition unit calculates, as the information, drive integration of the conveyance motor. The information indicating the time is acquired, and the conveyance stopping unit is configured to indicate that the temperature is lower than the predetermined value when the accumulated driving time does not exceed the predetermined time within the predetermined time. The sheet is stopped at the first position, and when the predetermined time is exceeded, the sheet is stopped at the second position on the assumption that the temperature shows a predetermined value or more.

  In addition, the sheets after the image formation on the first surface is accommodated up to M (plural) sheets on the path until the sheet returns to the image forming unit, and the sheets are stored one by one. Returning to the image forming unit, job execution means for executing a job for forming an image on the second surface is provided, and the job execution means is accommodated on the path when the temperature becomes a predetermined value or more during job execution. The number of sheets to be processed is switched to a predetermined number of N sheets less than M, and the conveyance stopping unit switches to the second position with respect to the sheet after switching after the number of sheets stored in the path is switched to N sheets. It is characterized by stopping.

  With this configuration, by temporarily stopping and waiting the sheet in the circulation path, it is possible to release the stop once and resume conveyance toward the image forming unit at the conveyance timing to the image forming unit. It is possible to reduce the sheet (paper) interval to ensure the productivity of image formation, and if the temperature of the image forming unit or its surroundings becomes a predetermined value or higher, the air flow path can be maintained even if the sheet is temporarily stopped. Since it is not blocked, the time during which the air passage is blocked can be reduced, and the temperature rise of the image forming unit can be suppressed as compared with the conventional case.

1 is a diagram illustrating an overall configuration of a printer. It is a block diagram which shows the structure of the control part with which a printer is equipped. FIG. 10 is an operation diagram illustrating a flow of a sheet in double-sided circulation conveyance. FIG. 6 is an operation diagram illustrating a flow of paper in double-sided alternate conveyance. FIG. 10 is an operation diagram illustrating a flow of a double-sided alternating sheet when the number of double-sided built-in sheets is three. FIG. 6 is a conceptual diagram illustrating the order of image formation on a first surface and a second surface of a sheet in double-sided alternate conveyance. 6 is a flowchart showing the contents of sheet conveyance control of a print job in a duplex alternate conveyance mode. It is a flowchart which shows the content of the subroutine of a timing process. 6 is a flowchart showing the contents of a subroutine for paper stop position setting processing. It is a schematic diagram which shows a mode that the rear end of the following paper is covering the discharge path among the two sheets accommodated in the circulation path. 10 is a flowchart showing the contents of a subroutine for paper conveyance stop processing. It is a flowchart which shows the content of the subroutine of 1st position stop control. It is a flowchart which shows the content of the subroutine of 2nd position stop control. FIG. 10 is a schematic diagram illustrating how a sheet is stopped by second position stop control. 10 is a timing chart for explaining a difference in image formation start timing between when the first position stop control is performed on the sheet S2 and when the second position stop control is performed. It is a flowchart which shows the content of the subroutine of a fan drive control process. It is a figure for demonstrating the content of the fan drive control process which concerns on a modification. It is a flowchart which shows the content of the fan drive control process which concerns on a modification. It is the schematic for demonstrating the structure of another cooling part which concerns on a modification. It is a flowchart which shows the content of the inversion position change process which concerns on a modification. It is another flowchart which shows the content of the inversion position change process which concerns on a modification. It is a figure which shows schematic structure of the conventional tandem type color printer.

Hereinafter, an embodiment of an image forming apparatus according to the present invention will be described using a tandem color digital printer (hereinafter simply referred to as “printer”) as an example.
<Overall configuration of printer>
FIG. 1 is a diagram illustrating the overall configuration of the printer 1.
As shown in the figure, the printer 1 includes an image forming unit 2, a feeding unit 3, a fixing unit 4, a duplex conveying unit 5, a control unit 6, an operation unit 7, a cooling unit 8, and the like. The printer has a double-sided printing function, is connected to a network, here a LAN, and receives a print job execution instruction from an external terminal device (not shown), and executes a print job based on the instruction. .

The print job includes a job in a single-sided mode for forming an image on one side of the paper S and a job in a double-sided mode for forming an image on each of the first and second sides of the paper S.
The image forming unit 2 includes image forming units (hereinafter referred to as “IU”) 10Y, 10M, 10C, corresponding to each of yellow (Y), magenta (M), cyan (C), and black (K). 10K and intermediate transfer belt 21 are provided.

The IUs 10Y to 10K include photosensitive drums 11Y to 11K that are rotationally driven in a direction indicated by an arrow A, charging units 12Y to 12K, exposure units 13Y to 13K, and developing units 14Y arranged around the photosensitive drums 11Y to 11K. It consists of ~ 14K.
The intermediate transfer belt 21 runs in the direction indicated by the arrow B, and the IUs 10Y, 10M, 10C, and 10K are arranged in this order from upstream to downstream in the circumferential direction of the intermediate transfer belt 21. Here, the IU 10K is located on the most downstream side.

The intermediate transfer belt 21 is stretched around a driving roller 22 and a driven roller 23.
Primary transfer rollers 24Y, 24M, 24C, and 24K are arranged at positions facing the photosensitive drums 11Y to 11K via the intermediate transfer belt 21, and at positions facing the drive roller 22 via the intermediate transfer belt 21. A secondary transfer roller 25 is disposed. A transfer nip is secured between the intermediate transfer belt 21 and the secondary transfer roller 25, and this transfer nip constitutes a secondary transfer position 29.

The feeding unit 3 includes a paper feed cassette 31 that stores the paper S, a feed roller 32 that feeds the paper S in the paper feed cassette 31 toward the transport path 39 one by one, and the fed paper S by an arrow C. Conveying roller pairs 33 and 34 for conveying in the direction shown, and a timing roller pair 35 for taking the timing of feeding the paper S to the secondary transfer position 29 are provided.
The fixing unit 4 includes a cylindrical fixing roller 41, a pressure roller 42 that presses the fixing roller 41 and secures a fixing nip N between the surface of the fixing roller 41, and a heater 43 that is inserted into the fixing roller 41. A temperature sensor 44 that detects the surface temperature of the fixing roller 41, and the like. The controller 6 controls the energization of the heater 43 based on the temperature detection result of the temperature sensor 44, so that the surface temperature of the fixing roller 41 becomes the fixing temperature. For example, the temperature is maintained at 160 ° C.

The double-sided conveyance unit 5 includes a discharge roller pair 50 and double-sided conveyance roller pairs 51, 52, 53, 54 provided in the circulation path 58.
The cooling unit 8 includes an IU cooling fan 81 and a blower path 82.
The air blowing path 82 is provided so as to cross the conveyance path 39 and the circulation path 58. Here, the conveyance path 39 is composed of a pair of guide plates 3a arranged to face each other with an interval necessary for paper conveyance, and each of the pair of guide plates 3a has a direction orthogonal to the paper conveyance direction. A notch (slit, hole, etc.) 3b for allowing the air flow to pass therethrough is provided, and a duct is provided outside the transport path 39 so that the air flow can cross the transport path 39 through the notch 3b. It is configured to be provided.

This configuration is the same for the circulation path 58. Needless to say, the cutout 3b is provided in a shape that does not affect the transportability of the paper S.
The air passage 82 is located in a space (hereinafter referred to as “IU peripheral space”) 19 having one end (inlet) between the conveyance path 39 and the IU 10 K, and the other end (outlet) is connected to the discharge port 85. It is configured to be.

The IU cooling fan 81 is disposed in the middle of the air passage 82. By driving the IU cooling fan 81, the air in the IU peripheral space 19 is sucked from the inlet of the air passage 82, and the sucked air passes through the air passage 82. It passes through the discharge port 85 and is discharged out of the machine.
As a result, when the temperature of the IU peripheral space 19 is high due to the heat of the paper S conveyed by double-sided printing, the heated air is excluded outside the apparatus, so that the IU 10Y and 10C including the IU 10K The temperature is prevented from continuing to rise (IU cooling).

The air flow (cooling air) flowing through the air passage 82 is conveyed through the circulation path 58 while the sheet S conveyed through the conveyance path 39 passes through the intersection position Xa where the conveyance path 39 and the air passage 82 intersect. Either the sheet S to be passed passes through the intersection position Xb where the circulation path 58 and the blower path 82 intersect, and the sheet S stops by closing the blower path 82 at that position.
The operation unit 7 is arranged on the front surface of the apparatus main body at a position where the user can easily operate. .

The control unit 6 receives an image signal transmitted from an external terminal device, converts the image signal into a digital image signal for Y to K colors, an image forming unit 2, a feeding unit 3, a fixing unit 4, The double-sided conveyance unit 5 and the like are controlled to execute image forming operations in the single-side mode and the double-side mode.
<Single-sided mode>
In the case of single-side mode image formation, the photosensitive drums 11Y to 11K rotating in the direction of arrow A are cleaned by a cleaner (not shown) for each of the IUs 10Y to 10K, and then uniformly charged by the charging units 12Y to 12K. The exposed surfaces of the photosensitive drums 11Y to 11K are exposed by the exposure units 13Y to 13K, so that electrostatic latent images based on the image data are formed on the surfaces of the photosensitive drums 11Y to 11K.

  The formed electrostatic latent image is developed with toner as a developer by the developing units 14Y to 14K to be visualized as a toner image. The color toner images on the photosensitive drums 11Y to 11K are primarily transferred from the photosensitive drums 11Y to 11K onto the intermediate transfer belt 21 by the action of electrostatic force by the primary transfer rollers 24Y to 24K. At this time, the image forming operations for the respective colors are executed at different timings so that the toner images are primarily transferred at the same position on the intermediate transfer belt 21.

Each color toner image primarily transferred onto the intermediate transfer belt 21 is moved to the secondary transfer position 29 by the intermediate transfer belt 21 that circulates in the direction indicated by the arrow B.
On the other hand, in accordance with the movement timing of each color toner image on the intermediate transfer belt 21, the sheet S is fed from the feeding unit 3 via the timing roller pair 35, and the sheet S circulates. The toner images are conveyed between the intermediate transfer belt 21 and the secondary transfer roller 25, and the toner images on the intermediate transfer belt 21 are collectively transferred onto the first surface of the sheet S by electrostatic force at the secondary transfer position 29. Next transferred.

The sheet S that has passed through the secondary transfer position 29 is conveyed to the fixing unit 4, and when passing through the fixing nip N, each color toner image on the sheet S is heated and pressed to be fixed on the first surface of the sheet S. Is done.
The sheet S that has passed through the fixing unit 4 is conveyed in the direction indicated by the arrow D and guided to the reversing claw 57.
The reversing claw 57 is configured to be switchable between a first posture shown by a solid line and a second posture shown by a broken line by swinging up and down around a fulcrum. Until the leading edge of the sheet S in the transport direction (the leading edge of the sheet) reaches the reversing claw 57, the reversing claw 57 takes the first posture under its own weight, and when the sheet S passes the reversing claw 57, the sheet S By pushing up the reverse claw 57, the reverse claw 57 is switched to the second posture. When the rear end of the sheet S in the transport direction (the rear end of the sheet) passes through the reversing claw 57, the reversing claw 57 returns to the original first posture due to its own weight.

The sheet S that has passed through the reversing claw 57 is further conveyed in the direction indicated by the arrow D, and is guided to the discharge roller pair 50 through the discharge path 56. The discharge roller pair 50 is rotated (normally rotated) in the direction indicated by the solid line in the drawing, and the sheet S conveyed through the discharge path 56 is further conveyed by the discharge roller pair 50 and discharged outside the apparatus. The discharged paper S is stored in the discharge tray 36.
Residual toner remaining on the surface of the intermediate transfer belt 21 after the secondary transfer is cleaned by the cleaner 26.

<In duplex mode>
In the case of image formation in the duplex mode, the sheet S that has passed the secondary transfer position 29 is conveyed from the fixing unit 4 to the discharge roller pair 50 through the reverse claw 57 and the discharge path 56.
At this time, the discharge roller pair 50 is rotating forward, and the sheet S is further conveyed in the direction indicated by the arrow D by the discharge roller pair 50.

  Immediately after the trailing edge of the paper S conveyed in the direction indicated by the arrow D by the discharge roller pair 50 passes through the reverse claw 57, the discharge roller pair 50 is driven in reverse. By the reverse rotation of the discharge roller pair 50, the sheet S is reversed and conveyed in the direction indicated by the arrow E (switchback). At this time, since the reverse claw 57 has returned to the first posture (solid line), the leading edge of the sheet S after the switchback is guided to the circulation path 58 while being guided above the reverse claw 57.

  Since the rear end of the sheet S is reversed immediately after passing through the reversing claw 57, it is necessary for the rear end of the sheet S to be guided to the circulation path 58 rather than being reversed after being conveyed to the position immediately before the discharge roller pair 50. The time can be shortened and the distance from the preceding paper can be reduced accordingly, and the productivity of paper conveyance is improved. It can be said that the discharge path 56 serves as both a discharge path for discharging the paper S and an inversion path for switching back the paper S.

The sheet S guided to the circulation path 58 is transported in the direction indicated by the arrow F by the double-sided transport roller pairs 51 to 54. The positions X0, X1, and X2 shown on the circulation path 58 will be described later.
The sheet S conveyed through the circulation path 58 is again conveyed to the secondary transfer position 29 via the timing roller pair 35 again. In accordance with the conveyance timing of the sheet S to the secondary transfer position 29, the image forming unit 2 forms each color toner image on the second surface, and each color toner image superimposed on the intermediate transfer belt 21 is formed. Secondary transfer is performed on the second surface of the sheet S at a time at the secondary transfer position 29.

  The sheet S on which the respective color toner images are secondarily transferred onto the second surface at the secondary transfer position 29 is conveyed to the fixing unit 4, and the respective color toner images are fixed on the second surface of the sheet S at the fixing unit 4. The sheet S that has passed through the fixing unit 4 is guided to the discharge roller pair 50 via the reverse claw 57. At this time, the discharge roller pair 50 is rotating forward in the same manner as in the single-sided mode, and the transported paper S is further transported and discharged outside the apparatus.

  In the transport path 39 of the paper S, a timing sensor 37 is provided at a position near the upstream side in the paper transport direction with respect to the timing roller pair 35, and a paper discharge sensor at a position near the upstream side in the paper transport direction with respect to the reversing claw 57. 38 is provided. In the circulation path 58, a paper detection sensor SE1 is provided at a position between the double-sided conveyance roller pair 51 and 52, and a paper detection sensor SE2 is located at a position nearer the upstream side in the paper conveyance direction than the double-sided conveyance roller pair 53. A sheet detection sensor SE3 is provided at a position between the pair of double-sided conveyance rollers 53 and 54.

Each sensor detects the leading edge and the trailing edge of the conveyed paper S and sends the detection signal to the control unit 6. For example, a reflective or transmissive optical sensor is used. Other types of sensors may be used as long as they can detect paper.
The drive motor 55 rotationally drives rollers such as the photosensitive drums 11Y to 11K, the intermediate transfer belt 21, the fixing roller 41 and the pressure roller 42, the conveying roller pair 33 of the feeding unit 3, and the timing roller pair 35. is there. Although the driving force of the driving motor 55 is transmitted to each roller pair, the specific roller pair, for example, the conveying roller pair 33, 34, the timing roller pair 35, etc., rotate and stop respectively. A clutch (not shown) is provided in the middle of the transmission path so that the roller pair can be individually switched between rotation and stop by the operation control of the clutch of the control unit 6.

The reverse motor 59 drives the discharge roller pair 50 by switching between normal rotation and reverse rotation. The double-sided conveyance motor 91 rotates the double-sided conveyance roller pairs 51 and 52. The double-sided conveyance motor 92 The double-sided conveying roller pair 53 and 54 are rotationally driven.
Around the IU 10K, a temperature detection sensor 18 for detecting the ambient temperature of the IU 10K (hereinafter referred to as “IU ambient temperature”) is disposed. A detection signal from the temperature detection sensor 18 is sent to the control unit 6.

The control unit 6 executes a paper stop position setting process (described later) for setting the stop position of the paper S in the circulation path 58 based on the detection signal from the temperature detection sensor 18.
<Configuration of control unit 6>
FIG. 2 is a block diagram illustrating a configuration of the control unit 6.
As shown in the figure, the control unit 6 includes, as main components, a communication interface (I / F) unit 101, a CPU 102, a ROM 103, a RAM 104, an image processing unit 105, an image memory 106, and a sheet. A stop position setting unit 107, a timing processing unit 108, a sheet conveyance stop control unit 109, a fan drive control unit 110, and the like are provided, and the respective units can communicate with each other.

A communication interface (I / F) unit 101 is an interface for connecting to a LAN such as a LAN card or a LAN board, receives print job data from the outside, and sends the received data to the image processing unit 105.
The image processing unit 105 converts the print job data from the communication I / F unit 101 into image data of reproduction colors Y to K and performs predetermined correction processing such as gradation correction on the image data, thereby The image data is stored for each reproduction color.

At the time of printing, the CPU 102 reads out image data from the image memory 106 and controls the operations of the image forming unit 2 and the double-sided conveyance unit 5 with timing, and prints in one-sided and double-sided modes based on the read image data. Make the job run smoothly.
Further, the CPU 102 drives and controls the drive motor 55, the reverse motor 59, the duplex conveyance motors 91 and 92, and the IU cooling fan 81.

  Further, the CPU 102 sets either double-sided circulation conveyance or double-sided alternate conveyance as the duplex mode according to the paper size used for printing. Here, double-sided circulation conveyance is set when large-size paper of A3 or larger is used, and double-sided alternating conveyance is set when medium-sized paper of B4 or smaller is used. Hereinafter, a method of conveying the paper S by the double-sided circulation conveyance and the double-sided alternate conveyance will be specifically described with reference to FIGS.

<Double-sided circulation conveyance>
FIG. 3 is an operation diagram showing the flow of the sheet S in the double-sided circulation conveyance.
As shown in the figure, in the double-sided circulation conveyance, when a plurality of sheets S1, S2,... Are continuously fed and double-sided printing is performed, the sheet S1 is fed (FIG. 3A). The second page image is formed on the first surface (FIG. 3B). The numerical value indicated by a circle in the figure indicates the number of pages. The same applies to other figures.

After the sheet S1 is switched back (FIG. 3C), it is conveyed through the circulation path 58 (FIG. 3D), and the first page image is formed on the second surface of the sheet S1 (FIG. 3E). In parallel with the discharge of the sheet S1, the next sheet S2 is fed (FIG. 3F), and the fourth page image is formed on the first surface of the sheet S2 (FIG. 3G). Operation is performed.
That is, image forming, fixing, switchback, circulation path conveyance, image forming, fixing, and discharging operations on the second surface are repeated for each sheet S.

<Double-sided alternate conveyance>
FIG. 4 is an operation diagram illustrating the flow of the sheet S in the double-sided alternate conveyance.
As shown in FIG. 4, in the double-sided alternate conveyance, when a plurality of sheets S1, S2,... Are continuously fed and double-sided printing is performed, the second page is displayed on the first side of the sheet S1 in FIG. In FIG. 4B, the sheet S2 is fed in parallel with the switchback of the sheet S1.

4C, the image of the fourth page is formed on the first surface of the sheet S2 in parallel with the operation of transporting the sheet S1 through the circulation path 58. In FIG. 4D, the switch of the sheet S2 is formed. Concurrently with the back, the sheet S1 is conveyed toward the timing roller pair 35.
In FIG. 4E, in parallel with the operation of forming the first page image on the second surface of the sheet S1, the sheet S3 after the switchback is conveyed through the circulation path 58 and the sheet S3 is fed. In FIG. 4F, the sheet S2 is further conveyed through the circulation path 58 in parallel with the discharge of the sheet S1.

  In FIG. 4G, while the image formation of the sixth page is performed on the first surface of the paper S3, the paper S2 in the circulation path 58 is temporarily at a position X1 where the leading edge has slightly passed through the duplex conveying roller pair 54. Stop. The reason for temporarily stopping is to prevent the sheets S3 and S2 from colliding on the transport path 39. The position X1 is set at the most downstream position of the circulation path 58 in the sheet conveyance direction so that the sheet S2 can be quickly reached by the timing roller pair 35 when the conveyance is resumed after the stop. This position X1 is the reference for the stop position.

Stopping the sheet S with the leading edge of the sheet S positioned at the position X1 is referred to as stopping the sheet S at the first position X1. This is the same for the position X2, and stopping the sheet S with the leading edge of the sheet S positioned at the position X2 is referred to as stopping the sheet S at the second position X2.
When the sheet S2 stops at the first position X1, the trailing edge of the sheet S2 is applied to the air passage 82, and the air passage 82 is blocked while the paper S2 is temporarily stopped. The blockage of the air passage 82 causes the temperature around the IU to rise.

If the IU ambient temperature is low to some extent, the toner in the developing units 14Y to 14K does not melt as long as the air passage 82 is blocked while the sheet S2 is temporarily stopped, but is somewhat high. If the air passage 82 is blocked in this state, the toner may melt due to further temperature rise.
Therefore, in the present embodiment, if the IU ambient temperature is equal to or higher than the predetermined temperature, the stop position of the sheet S2 is downstream of the first position X1 in the sheet conveyance direction, and the second position X2 (see FIG. 14 (g1)), and control is performed so as to prevent the air blowing path 82 from being blocked by the waiting sheet S2. Details of this control will be described later.

In FIG. 4H, the conveyance of the sheet S2 is resumed at the timing when the image formation on the first surface of the sheet S3 is finished, the leading edge of the sheet S2 reaches the timing roller pair 35, and the leading edge of the sheet S2 is reached. When a predetermined amount of loop is formed in the part, the sheet S2 is temporarily stopped.
In FIG. 4I, in parallel with the switchback of the sheet S3, the sheet S2 is conveyed toward the secondary transfer position 29 by the timing roller pair 35.

  In FIG. 4 (j), in parallel with the operation of forming the image of the third page on the second surface of the sheet S2, the sheet S3 after the switchback is transported through the circulation path 58 and the sheet S4 is fed. In FIG. 4K, the sheet S3 is further conveyed through the circulation path 58 in parallel with the discharge of the sheet S2. In FIG. 4L, while the image formation of the eighth page is performed on the first surface of the sheet S4, the sheet S3 in the circulation path 58 has a first position X1 at which the leading edge slightly passes the duplex conveying roller pair 54. Stop at once. This is the same operation as once stopping the sheet S2.

Thereafter, the operations shown in FIGS. 4H to 4L are repeatedly performed for each sheet.
FIG. 6A is a conceptual diagram illustrating the order of image formation on the first surface and the second surface of the sheet in the double-sided alternate conveyance illustrated in FIG. This figure shows an example in which the number of sheets is four.
As shown in the figure, image formation is performed in the order of pages 2, 4, 1, 6, 3, 8, 5, 7 on the sheets S1 to S4, and at the first, sixth, third, and eighth pages, As the paper transport destination, the discharge and the circulation path are alternately switched, which indicates that the sheet S from the paper feed cassette 31 and the sheet S from the circulation path 58 are alternately switched.

As described above, in the double-sided alternating conveyance, the paper S is alternately conveyed from the paper feed cassette 31 and the circulation path 58 to the secondary transfer position 29, and image formation on the first surface is performed on the paper S from the paper feed cassette 31. Then, after fixing and switching back, the sheet S is guided (not discharged) to the circulation path 58, and the operation of forming, fixing, and discharging the image on the second surface is repeatedly performed for the sheet S from the circulation path 58.
Since the double-sided alternate conveyance is a method of feeding the next sheet S from the paper feed cassette 31 in a state where one or more sheets S are accommodated in the circulation path 58, the circulation path 58 is similar to the double-sided circulation conveyance (FIG. 3). The productivity of double-sided printing can be improved as compared with the method of feeding the next sheet S from the sheet feeding cassette 31 in a state where no sheet S is accommodated therein. Note that in this embodiment, double-sided circulation conveyance is performed for large-sized sheets having a long length in the sheet conveyance direction without applying double-sided alternate conveyance due to structural restrictions such as the length of the circulation path 58. Yes.

  FIG. 4 illustrates a state in which the sheet S after the image formation on the first surface is performed using the medium size (for example, A4) sheet in the duplex alternate conveyance until the sheet S returns to the image forming unit 2. In this example, double-sided printing is performed while accommodating a maximum of two sheets S on the path (conveying path 39, discharge path 56, and circulation path 58). . This double-sided built-in number is determined from the size of the paper S (the length in the paper conveyance direction), the length of the conveyance path 39, the discharge path 56, the circulation path 58, and the like, but in the case of a small size (for example, B5) paper, Three is set.

FIG. 5 is an operation diagram showing the flow of the double-sided alternating transport sheet S when the number of both-side built-in sheets is three.
In FIG. 5A, after the image of the second page is formed on the first surface of the conveyed paper S1, in FIG. 5B, the first of the paper S2 is performed in parallel with the switchback of the paper S1. An image of the fourth page is formed on the surface. In FIG. 5C, the sheet S2 is switched back in parallel with the operation of transporting the sheet S1 after the switchback through the circulation path 58, and an image of the sixth page is formed on the first surface of the sheet S3. Is done.

In FIG. 5D, in parallel with the conveyance of the sheet S1 to the timing roller pair 35, the sheet S2 is conveyed through the circulation path 58 and the sheet S3 is switched back.
In FIG. 5E, the sheet S3 is conveyed through the circulation path 58 in parallel with the operation of forming the first page image on the second surface of the sheet S1. At this time, the sheet S4 is fed following the sheet S1, and the sheet S2 in the circulation path 58 is temporarily stopped at the first position X1 where the leading edge slightly passes through the double-sided conveying roller pair 54. This temporary stop is the same as in FIG.

In FIG. 5F, in parallel with the discharge of the sheet S1, the image of the eighth page is formed on the first surface of the sheet S4, and the sheet S3 is conveyed through the circulation path 58. The conveyance of the sheet S2 is resumed at the timing when the image formation on the first surface of the sheet S4 ends, and the sheet S2 is conveyed toward the secondary transfer position 29.
In FIG. 5G, the image of the third page is formed on the second surface of the sheet S2 in parallel with the switchback of the sheet S4. At this time, the sheet S5 is fed following the sheet S2, and the sheet S3 in the circulation path 58 is temporarily stopped at the first position X1.

In FIG. 5H, in parallel with the discharge of the sheet S2, an image of the tenth page is formed on the first surface of the sheet S5, and the sheet S4 is conveyed through the circulation path 58. The conveyance of the sheet S3 is resumed at the timing when the image formation on the second surface of the sheet S5 ends, and the sheet S3 is conveyed toward the secondary transfer position 29.
In FIG. 5I, the image of the fifth page is formed on the second surface of the sheet S3 in parallel with the switchback of the sheet S5. At this time, the sheet S4 in the circulation path 58 is temporarily stopped at the first position X1.

In FIG. 5J, in parallel with the discharge of the sheet S3, an image of the seventh page is formed on the second surface of the sheet S4, and the sheet S5 is conveyed through the circulation path 58. In FIG. 5 (k), the sheet S5 is conveyed toward the secondary transfer position 29 in parallel with the discharge of the sheet S4. In FIG. 5 (l), the image of the ninth page is displayed on the second surface of the sheet S5. After the formation, the sheet S5 is discharged.
FIG. 6B is a conceptual diagram showing the order of image formation on the first side and the second side of the sheet in the double-sided alternating conveyance when the number of both-side built-in sheets is three.

As shown in the figure, images are formed in the order of pages 2, 4, 6, 1, 8, 3, 10, 5, 7, 9 on the sheets S1 to S5, and 1, 8, 3, 10 At the fifth page, as the paper transport destination, the discharge and the circulation path are alternately switched, and the paper S from the paper feed cassette 31 and the paper S from the circulation path 58 are alternately switched.
Since the number of sheets S contained in the circulation path 58 is larger in the case of three double-sided built-in sheets than in the case of two sheets, the productivity of double-sided printing is further improved. In the present embodiment, during a print job, the number of double-sided built-in sheets is set to either 2 or 3 depending on the paper size used.

Returning to FIG. 2, the ROM 103 stores a control program related to the image forming operation, and the RAM 104 is used as a work area of the CPU 102.
The paper stop position setting unit 107 detects the IU peripheral temperature in the double-sided alternate conveyance, and based on the detected IU peripheral temperature, the stop position of the paper S in the circulation path 58 is set to either the first position X1 or the second position X2. Set it.

The timing processing unit 108 controls the rotation and stop of the timing roller pair 35.
The sheet conveyance stop control unit 109 temporarily transfers the sheet S conveyed through the circulation path 58 to the first position X1 or the second position X2 set by the sheet stop position setting unit 107 during execution of the double-sided alternate conveyance. Stop and then resume transport.
The fan drive control unit 110 drives and controls the IU cooling fan 81.

<Paper transport control using double-sided alternate transport mode>
FIG. 7 is a flowchart showing the contents of the paper conveyance control of the print job in the duplex duplex conveyance mode. This control is executed by the control unit 6 in units of print jobs.
In the feeding process (step S1) shown in the figure, the sheet S of the number designated by the user for the job is fed out from the sheet cassette 31 by the feed roller 32 one by one. Here, simultaneously with the feeding of the paper S, the transport roller pairs 33 and 34 are also rotated.

  In this feeding process, every time one sheet S is fed out, a management table for the sheet S is created. The management table is a table that stores information such as the feeding order (how many sheets are used), the sheet size, the transport destination (discharge or circulation path), and the like. Here, the conveyance destination is written with a circulation path between the time when the sheet S is fed out and before the switchback, and is updated to discharge after the switchback.

The timing process (step S <b> 2) is a process that takes a timing for feeding the fed paper S to the secondary transfer position 29 by the timing roller pair 35, and is executed by the timing processing unit 108.
The paper discharge process (step S3) is a process of discharging the paper S that has been transported from the transport path 39 to the discharge roller pair 50 via the discharge path 56, and refers to the above management table to transport the paper S. If the tip is discharged, the sheet S is discharged by the discharge roller pair 50.

  The switchback process (step S4) is a process of conveying the sheet S from the discharge path 56 to the circulation path 58 by reversing the discharge roller pair 50 when the conveyance destination of the sheet S is the circulation path 58. The timing of reverse rotation of the discharge roller pair 50 is the time when a predetermined time has elapsed after the trailing edge of the paper S is detected by the paper discharge sensor 38. This predetermined time is, for example, the time required for the trailing edge of the paper S to pass through the reverse claw 57 after passing the detection position of the paper discharge sensor 38.

The circulation path conveyance process (step S5) is a process in which the sheet S conveyed to the circulation path 58 is conveyed on the circulation path 58 by the double-side conveyance roller pairs 51 to 54.
The paper stop position setting process (step S6) is a process for setting the stop position of the paper S being conveyed through the circulation path 58 to either the first position or the second position based on the detected IU ambient temperature. Yes, executed by the paper stop position setting unit 107.

The paper transport stop process (step S7) is a process for temporarily stopping the paper S at the set stop position, and is executed by the paper transport stop control unit 109.
The fan drive control process (step S8) is a process for switching the rotation and stop of the IU cooling fan 81 in accordance with the stop position of the paper S, and is executed by the fan drive control unit 110.
The CPU 102 generally processes each process at a time required to execute each process on the sheet stop position setting unit 107, the timing processing unit 108, the sheet conveyance stop control unit 109, and the fan drive control unit 110. Is instructed to execute. Therefore, each process may be performed in parallel.

<Timing processing>
FIG. 8 is a flowchart showing the contents of a timing processing subroutine.
If it is determined that one sheet S has been fed (“YES” in step S11), it is determined whether or not the leading edge of the sheet S transported on the transport path 39 has been detected by the timing sensor 37 (step S12). ). This determination is made based on whether or not a level change (on-edge) is detected when the signal level switches from L to H when the timing sensor 37 detects the leading edge of the sheet. The method for detecting the leading edge of the paper S is similarly used in other processes.

  If it is determined that the leading edge of the sheet S has been detected (“YES” in step S12), it is determined whether or not a predetermined time Ta has elapsed since the detection (step S13). Here, for a predetermined time Ta, the leading edge of the sheet S is continued by the conveyance of the sheet S by the conveying roller pairs 33 and 34 while the leading edge of the sheet S is in contact with the timing roller pair 35 in the stopped state. This corresponds to the time required to form a loop of a predetermined size in the part. The reason why the loop having a predetermined size is formed is to correct the skew of the paper S.

The predetermined time Ta is obtained in advance from experiments or the like, and the data is stored in the ROM 103 or the like. The predetermined time Ta is measured by an internal timer (not shown). It should be noted that the fact that the predetermined time is obtained in advance from experiments or the like and that the time measurement is performed by the internal timer is similarly applied to other predetermined times such as the predetermined time Tb described later.
If it is determined that the predetermined time Ta has elapsed (“YES” in step S13), the rotation of the conveying roller pairs 33 and 34 is stopped (step S14).

  Then, the standby flag is set to ON (step S15). This standby flag indicates that the sheet S in the circulation path 58 needs to be waited in the circulation path 58, that is, if the sheet S in the circulation path 58 is transported to the timing roller pair 35 without waiting. 39 is a flag indicating that standby is required because the sheet S collides with the sheet S existing on the sheet 39 or is too close. This standby flag is referred to in the paper conveyance stop process (step S7).

  In step S16, it is determined whether or not the conveyance timing for sending the paper S to the secondary transfer position 29 has been reached. If the sheet S is conveyed by the timing roller pair 35 at this timing, the conveyance timing is determined when the leading edge of the sheet S reaches the secondary transfer position 29 and the intermediate transfer belt of the image to be formed on the sheet S. This corresponds to the timing when the leading edge of the image forming area on 21 reaches the secondary transfer position 29.

  Here, the leading edge of the image forming area on the intermediate transfer belt 21 after the start of image formation is longer than the time α required for the leading edge of the sheet S to reach the secondary transfer position 29 after the rotation of the timing roller pair 35 starts. The time β required to reach the secondary transfer position 29 is longer, and image formation is started first, and at the time γ when the time indicating the difference between the times α and β has elapsed, Transport of S to the secondary transfer position 29 is started. This time γ corresponds to the conveyance timing. Until the conveyance timing is reached, the sheet S is in a state of waiting for conveyance by the timing roller pair 35.

  Depending on the device configuration, the time α may be longer than the time β. In this case, the conveyance of the sheet S by the timing roller pair 35 is started first, and image formation is started at time γ. For example, a case where a monochrome image of only black (K) color is formed by the IU 10K located at the most downstream in the circumferential direction of the intermediate transfer belt 21 or a case of printing by an image forming apparatus capable of forming only a monochrome image is assumed. If the times α and β are the same length, the conveyance of the sheet S and the image formation by the timing roller pair 35 are started simultaneously.

If it is determined that the conveyance timing has been reached (“YES” in step S16), rotation of the timing roller pair 35 is started (step S17).
It is determined whether or not the trailing edge of the sheet S conveyed by the timing roller pair 35 has been detected by the timing sensor 37 (step S18). This determination is made based on whether or not a level change (off edge) is detected when the signal level is switched from H to L when the timing sensor 37 detects the trailing edge of the sheet. The method for detecting the trailing edge of the paper S is similarly used in other processes.

When it is determined that the trailing edge of the sheet S has been detected (“YES” in step S18), it is determined whether or not a predetermined time Tb has elapsed since the detection (step S19). Here, the predetermined time Tb corresponds to the time required for the trailing edge of the sheet S to pass the timing roller pair 35 after the trailing edge of the sheet S is detected.
If it is determined that the predetermined time Tb has elapsed ("YES" in step S19), the rotation of the timing roller pair 35 is stopped (step S20), the standby flag is switched off (OFF) (step S21), and the process returns. .

The standby flag is on after the sheet S reaches the timing roller pair 35 and passes through the timing roller pair 35.
<Paper stop position setting process>
FIG. 9 is a flowchart showing the contents of a subroutine for the paper stop position setting process.
First, the IU ambient temperature is detected (step S31). The detection of the IU ambient temperature is performed based on the temperature detection result by the temperature detection sensor 18.

  It is determined whether or not the IU ambient temperature is lower than a predetermined value (step S32). Here, the predetermined value is a threshold value for determining whether the stop position of the sheet S in the circulation path 58 is set to the first position or the second position, and the toner melting temperature in the developing units 14Y to 14K. In advance. Specifically, when the stop position of the paper S is set to the first position, a temperature at which the temperature of at least one of the developing units 14Y to 14K is likely to approach the toner melting temperature is obtained from an experiment or the like. Data is stored in the ROM 103 or the like.

If it is determined that the IU ambient temperature is lower than the predetermined value (“YES” in step S32), the stop position of the sheet S in the circulation path 58 is set to the first position X1 (step S33), and the process returns. If it is determined that the IU ambient temperature is equal to or higher than the predetermined temperature ("NO" in step S32), it is determined whether the number of double-sided built-in sheets is three (step S34).
If it is determined that the double-sided built-in number is not three, that is, two (“NO” in step S34), the stop position of the paper S in the circulation path 58 is set to the second position X2 (step S35). To return.

If it is determined that the double-sided built-in number is 3 (“YES” in step S34), the setting of the double-sided built-in number is switched from 3 to 2 (step S36), and the process proceeds to step S35. Also in this case, the stop position of the sheet S is set to the second position X2. The reason for switching the number of double-sided built-in sheets from 3 to 2 is as follows.
That is, when the sheet S2 is stopped at the second position X2 as shown in FIG. 10, the trailing edge S3a of the sheet S3 is placed on the discharge path 56 due to the relationship between the length of the circulation path 58 and the sheet interval between the following sheet S3. It will be left in.

The sheet S3 needs to be conveyed to the circulation path 58 through the discharge path 56 in order to avoid collision with the sheet S1 until the sheet S1 to be discharged enters the discharge path 56.
However, in the present embodiment, there is no time allowance between the sheet S3 passing through the discharge path 56 and the sheet S1 entering the discharge path 56, and there is a possibility that the sheet S1 will be hit too close. When the stop position is set to the second position X2, the number of double-sided built-in sheets is switched from 3 to 2 to ensure good conveyance of the paper.

  When switching the number of double-sided built-in sheets during execution of a print job, the image is formed on the currently transported paper S, the paper S is discharged, and the next new paper S is fed from the paper feed cassette 31. Then, printing according to the number of double-sided built-in sheets after switching, here two sheets, is started. This operation is handled by the CPU 102, and when performing this operation, the CPU 102 functions as a job execution unit that performs duplex printing by switching the number of both-side built-in sheets.

<Paper transport stop processing>
FIG. 11 is a flowchart showing the contents of a subroutine for paper conveyance stop processing.
It is determined whether or not the leading edge of the sheet S guided to the circulation path 58 after the switchback has reached the position X0 in the circulation path 58 (step S51). This determination is made based on whether or not a predetermined time Tc preset as a time required for the leading edge of the paper S to reach the position X0 after passing the detection position by the paper detection sensor SE1 has passed.

This position X0 is a position in the vicinity of the upstream side in the sheet conveyance direction with respect to the double-sided conveyance roller pair 53 on the circulation path 58 as shown in FIG. 1, and the sheet S is stopped at the first position X1 or the second position. This corresponds to a determination point for determining whether to stop at X2.
Returning to FIG. 11, when it is determined that the leading edge of the sheet S has reached the determination position X0 (“YES” in step S51), it is determined whether the standby flag is on or off (step S52).

If it is determined that the standby flag is off (“NO” in step S52), the process ends and the process returns. In this case, the sheet S in the circulation path 58 is transported toward the timing roller pair 35 without being temporarily stopped (corresponding to FIGS. 4C and 4D).
On the other hand, if it is determined that the standby flag is on (“YES” in step S52), it is determined whether or not the stop position set in the paper stop position setting process is the first position X1 (step S53).

If it is determined that the stop position is set to the first position X1 (“YES” in step S53), the first position stop control is executed (step S54).
FIG. 12 is a flowchart showing the contents of the first position stop control subroutine.
As shown in the figure, it is determined whether or not the leading edge of the paper S on the circulation path 58 has been detected by the paper detection sensor SE3 (step S151).

If it is determined that the leading edge of the paper S has been detected (“YES” in step S151), it is determined whether or not a predetermined time Td has elapsed since the detection (step S152). Here, the predetermined time Td corresponds to the time required for the leading edge of the paper S to reach the first position X1 on the circulation path 58 after passing the detection position by the paper detection sensor SE3.
When it is determined that the predetermined time Td has elapsed ("YES" in step S152), the rotation of the duplex conveyance motors 91 and 92 is stopped (step S153). Return. Accordingly, the sheet S is stopped when it is transported to the first position X1. This first position stop control corresponds to the control shown in FIGS. 4 (g) and 4 (l).

Returning to FIG. 11, in step S56, it is determined whether or not the standby flag is off. Until the standby flag is switched from on to off (“NO” in step S56), the paper S is temporarily stopped. When the standby flag is switched off (“YES” in step S56), the paper S once stopped. Is resumed (step S57), and the process returns.
Since the standby flag is switched from on to off after the sheet S from the sheet feeding cassette 31 passes through the timing roller pair 35 as described above, the sheet S is fed from the circulation path 58 to the timing roller pair 35 when the sheet is turned off. It can be transported toward. This control corresponds to the control shown in FIG.

On the other hand, when it is determined in step S53 that the set position is not the first position X1 but the second position X2 (“NO” in step S53), second position stop control is executed (step S55).
FIG. 13 is a flowchart showing the contents of a second position stop control subroutine.
As shown in the figure, it is determined whether or not the leading edge of the paper S on the circulation path 58 has been detected by the paper detection sensor SE2 (step S161).

If it is determined that the leading edge of the sheet S has been detected (“YES” in step S161), it is determined whether or not a predetermined time Te has elapsed since the detection (step S162). Here, the predetermined time Te corresponds to the time required for the leading edge of the paper S to reach the second position X2 on the circulation path 58 after passing the detection position by the paper detection sensor SE2.
If it is determined that the predetermined time Te has elapsed (“YES” in step S162), the rotation of the duplex conveyance motors 91 and 92 is stopped (step S163), and the process returns. Accordingly, the sheet S is stopped when it is transported to the second position X2.

FIG. 14 is a schematic diagram showing how the sheet S is stopped by the second position stop control. When the second position stop control is performed, FIGS. 14 (f) to 14 (g 1) and FIG. 14 (g 2). 14 (h1), the sheet is conveyed in the order of FIG. 4 (j).
As shown in FIG. 14G1, when the sheet S2 stops at the second position X2, the air passage 82 is not blocked by the sheet S2. At this time, since the conveyance of the sheet S3 by the timing roller pair 35 has not yet started, the air passage 82 is not blocked by the sheet S3, and the heated air in the IU peripheral space 19 passes through the air passage 82. The temperature rise of IU 10Y to 10K is prevented.

The conveyance of the sheet S3 by the timing roller pair 35 is started when the above-described time γ has elapsed since the formation of the image of the sixth page to be formed on the first surface of the sheet S3 is started.
Since the conveyance of the sheet S3 is started and the air passage 82 is opened until the leading edge of the paper S3 reaches the intersection position Xa of the conveyance path 39 and the air passage 82, the cooling of the IUs 10Y to 10K continues until then. Is done.

When the conveyed paper 3 passes through the timing roller pair 35, the standby flag is switched from on to off.
Returning to FIG. 11, in step S56, it is determined whether or not the standby flag has been switched off. If it is determined that the sheet is switched off (“YES” in step S56), the conveyance of the sheet S stopped at the second position X2 is restarted (step S57), and the process returns.

FIG. 14 (g2) and FIG. 14 (h1) show how the paper S2, which has been stopped at the second position X2, is resumed.
In the second position stop control, the sheet S2 is stopped at the second position X2, so that the sheet S2 arrives at the timing roller pair 35 later than when the sheet S2 is stopped at the first position X1.
Specifically, in the example of FIG. 4H of the first position stop control, the leading edge of the sheet S2 reaches the timing roller pair 35 just before the sheet S3 passes the secondary transfer position 29. In the example of FIG. 14 (h1) of the second position stop control, the leading edge of the sheet S2 has reached only the duplex conveying roller pair 54 immediately before the sheet S3 passes the secondary transfer position 29. The distance from the duplex conveying roller pair 54 to the timing roller pair 35 is delayed by the time Tf required for the sheet S2 to convey.

This time Tf is obtained in advance, and when the second position stop control is executed, the start of the third page image formation to be formed on the second surface of the sheet S2 is based on the timing of the first position stop control. In addition, the image formation start timing is changed so as to be delayed by the time Tf.
As described above, in the present embodiment, for each sheet, the sheet S is conveyed by the timing roller pair 35 when a certain time γ has elapsed from the start of image formation of the image to be formed on the sheet S. Since the start of the image formation is delayed by the time Tf, the subsequent operation is shifted later from the timing of the first position stop control by the time Tf. The transport to the secondary transfer position 29 is also started after the same time Tf after the first position stop control.

  Note that the image formation of the sixth page image on the first surface of the preceding sheet S3 is started before this delay control, and is excluded from this control, so the sheet S3 stops at the first position. It is conveyed by the timing roller pair 35 at the same timing as the control. As a result, the paper interval between the preceding paper S3 and the paper S2 to be delayed is made larger than in the first position stop control. In this case, the time point when the sheet S3 reaches the determination position X0 is delayed by a time Tf in order to match the sheet feeding timing of the next sheet S4. For example, only the sheet S3 is controlled such that the time required for switchback is delayed by the time Tf or the conveyance speed is decreased until the determination position X0 in the circulation path 58 is reached. These controls are performed by the CPU 102.

FIG. 15 is a timing chart for explaining the difference in image formation start timing between when the first position stop control is performed on the sheet S2 and when the second position stop control is performed.
As shown in the figure, when the sheet S3 is fed from the sheet feeding cassette 31 (time t1) (corresponding to the state of FIG. 4E), the leading edge of the sheet S3 is detected by the timing sensor 37 (time point). t2). The leading edge of the sheet S3 is conveyed to the timing roller pair 35. At this time, the timing roller pair 35 is stopped, and when a predetermined amount of loop is formed at the leading edge of the sheet S3, the sheet S3 is conveyed. Pause.

Image formation of the image of the sixth page on the first surface of the sheet S3 is started (time t3).
The determination that the image formation start timing has come is made when a predetermined event occurs in a state where image formation can be started. For example, it is determined that the leading edge of the sheet S3 has been detected by the timing sensor 37. It can be an event occurrence. The same applies to the sheet 2.

After a time Tγ from the start of the image formation, conveyance of the sheet S3 to the secondary transfer position 29 is started by the timing roller pair 35 (time point t4).
In parallel with the transport operation of the paper S3, the paper S2 is transported through the circulation path 58 (corresponding to the state of FIG. 4F). When the first position stop control is executed for the sheet S2, the sheet S2 is temporarily stopped at the first position X1 on the circulation path 58 (corresponding to the state shown in FIG. 4G).

When the stop of the sheet S2 is released and the conveyance is resumed (time t5), the leading edge of the sheet S2 is detected by the timing sensor 37 (time t6). When the leading edge of the sheet S2 is conveyed to the timing roller pair 35 and a predetermined amount of loop is formed at the leading end of the sheet S3, the conveyance of the sheet S3 is temporarily stopped (corresponding to the state of FIG. 4 (h)).
Image formation of the third page image on the first surface of the sheet S2 is started (time t7), and after a time Tγ, the conveyance of the sheet S2 to the secondary transfer position 29 is started by the timing roller pair 35 (time t8). ).

On the other hand, when the second position stop control is executed, the sheet S2 is temporarily stopped at the second position X2 on the circulation path 58 (corresponding to the state of FIG. 14 (g1)).
When the stop of the sheet S2 is released and the conveyance is resumed (time t5), the leading edge of the sheet S2 is detected by the timing sensor 37 (time t11). This time point t11 corresponds to a time point delayed by time Tf from the time point t6 in the first position stop control. When a predetermined amount of loop is formed at the leading end of the sheet S2, the conveyance of the sheet S2 is temporarily stopped.

  Image formation of the third page image on the first surface of the sheet S2 is started (time t12), and after a time Tγ, the conveyance of the sheet S2 to the secondary transfer position 29 is started by the timing roller pair 35 (time t13). ). Time points t12 and t13 correspond to time points delayed by time Tf from time points t7 and t8 in the first position stop control. Due to this delay control, there is a difference in image formation start timing between the first position stop control and the second position stop control.

In the above description, when the standby flag is OFF in FIG. 11 (“NO” in step S52), the sheet S in the circulation path 58 is not temporarily stopped. However, the present invention is not limited to this. For example, the sheet S is temporarily set to the first position X1. It may be configured to stop.
In other words, when the standby flag is OFF, the sheet S can be conveyed toward the timing roller pair 35. Therefore, even if the sheet S is temporarily stopped at the first position X1 in the circulation path 58, the sheet S is immediately stopped. The conveyance can be resumed and sent to the timing roller pair 35. The fact that the conveyance can be resumed immediately means that even if the conveyance is stopped at the first position X1, it takes only a very short time to block the air passage 82 by the paper S.

  For example, in the example of FIG. 4C, when the leading edge of the sheet S1 reaches the determination position X0, the sheet S2 has already passed the timing roller pair 35, and the standby flag is off. In this case, the sheet S1 is temporarily stopped at the first position X1, and if there is no trouble such as a jam in the image forming unit 2 or the feeding unit 3, the sheet S1 is immediately conveyed (for example, after several tens of milliseconds). It can be resumed and transported to the timing roller pair 35, and the trouble can be confirmed by temporarily stopping.

Note that if the number of double-sided built-in sheets is switched from 3 to 2 during execution of the print job (step S36), the sheet S is fed in accordance with the image forming operation in which the double-sided built-in number is 2 as described above. Since the conveyance is performed, the second position stop control is executed on the sheet S after the switching after the image forming operation by the switching is performed.
<Fan drive control processing>
FIG. 16 is a flowchart showing the contents of a subroutine of fan drive control processing.

The IU cooling fan 81 is driven (step S71). Thus, if the air passage 82 is not blocked by the paper S, the heated air in the IU peripheral space 19 (FIG. 1) is discharged outside the apparatus through the air passage 82. Thereby, cooling of IU10Y-10K is performed.
It is determined whether or not the paper S is stopped at the first position X1 in the duplex printing (step S72). This determination is made by determining that the period from the execution of step S153 to the execution of step S57 in the paper conveyance stop process described above.

If it is determined that the sheet S is not stopped at the first position X1 (“NO” in step S72), the process returns. If it is determined that the sheet S is stopped at the first position X1 (“YES” in step S72), the IU cooling fan 81 is stopped (step S73). This is done for the following reason.
That is, when the sheet S stops at the first position X1, the air passage 82 is blocked by the rear end portion of the sheet S. Even if the driving of the IU cooling fan 81 is continued in this state, the heated air in the IU peripheral space 19 cannot be discharged to the outside through the air passage 82, and the IU cooling fan 81 is loaded. This is to prevent the heated air from returning to the IU peripheral space 19 due to a reverse flow of air in the air passage 82.

  It is determined whether or not the conveyance of the paper S that has been stopped at the first position X1 has been resumed (step S74). Whether the conveyance of the sheet S has been resumed is determined by executing step S57 described above. Until the conveyance of the paper S is resumed (“NO” in step S74), the IU cooling fan 81 is stopped. When the conveyance of the paper S is resumed (“YES” in step S74), the IU cooling fan 81 is turned off. Drive (step S75) and return.

  There may be a configuration in which it is not necessary to stop the IU cooling fan 81, for example, the air flow does not reverse even if the IU cooling fan 81 is continuously driven. In such a configuration, the IU cooling fan 81 is not stopped. Alternatively, the driving may be continued. Each process of the sheet feeding process (step S1) to the fan drive control process (step S8) is executed for each sheet fed out from the sheet feeding cassette 31 during execution of the print job.

  As described above, in the present embodiment, the sheet S is temporarily stopped and waited in the circulation path 58 in order to take a timing to send the sheet S being conveyed through the circulation path 58 toward the image forming unit 2. In the configuration, when the sheet S is in the double-sided alternating conveyance mode, (a) if the IU peripheral temperature is lower than a predetermined value, the position is the position corresponding to the most downstream in the sheet conveyance direction of the circulation path 58, and the blower path by the stopped sheet S (B) If the IU ambient temperature is equal to or higher than a predetermined value, the air flow is routed upstream of the first position in the paper conveyance direction and the circulation path 58 through the air flow path 58. The leading edge of the sheet S is positioned upstream of the intersecting position Xb crossed by the sheet 82 in the sheet conveying direction, and the sheet S is stopped at the second position X2 where the blowing path 82 is not blocked.

  Thereby, when the IU ambient temperature is lower than the predetermined value, the sheet S stops at the first position X1 in the circulation path 58, and the first position X1 is closer to the timing roller pair 35 than the second position X2. When the conveyance of the sheet S in the circulation path 58 is resumed, the sheet S can be quickly reached by the timing roller pair 35 to start the image formation of the sheet S earlier, and the number of images formed per unit time can be increased. Increases the productivity of double-sided printing.

In this case, the air passage 82 is blocked by the paper S by stopping at the first position X1, but the temperature of the IUs 10Y to 10K is low, so that the toner melts even if the temperature rises to some extent. It will not reach.
On the other hand, when the IU ambient temperature becomes equal to or higher than the predetermined value, the paper S stops at the second position X2, so that the air passage 82 is not blocked and the heated air in the IU peripheral space 19 is blown. 82, the temperature rise of IU10Y to 10K is prevented.

  In this case, since the second position X2 is located upstream of the first position X1 in the paper conveyance direction, the time required for the paper S in the circulation path 58 that has been resumed to reach the timing roller pair 35 is the first time. Compared with the case of stopping at the first position X1, the time is increased by the time Tf, and the start of image formation on the first surface of the paper S is delayed. The productivity of double-sided printing is reduced by this delay time, but the temperature of IU 10Y to 10K is prevented from rising to the temperature range where the toner melts.

  The present invention is not limited to the image forming apparatus, and may be a method of conveying a sheet by switching the stop position of the sheet in the circulation path 58 between the first position X1 and the second position X2. Furthermore, the method may be a program executed by a computer. The program according to the present invention includes, for example, a magnetic disk such as a magnetic tape and a flexible disk, an optical recording medium such as a DVD-ROM, DVD-RAM, CD-ROM, CD-R, MO, and PD, and a flash memory recording medium. It can be recorded on various computer-readable recording media, and may be produced, transferred, etc. in the form of the recording medium, wired and wireless various networks including the Internet in the form of programs, In some cases, the data is transmitted and supplied via broadcasting, telecommunication lines, satellite communications, or the like.

(Modification)
As described above, the present invention has been described based on the embodiment. However, the present invention is not limited to the above-described embodiment, and the following modifications may be considered.
(1) In the above embodiment, the IU cooling fan 81 is stopped only while the paper S is stopped at the first position. However, the air passage 82 is further blocked by the paper S conveyed through the circulation path 58. During the peeling, the fan drive control process for stopping the IU cooling fan 81 may be performed regardless of the stop position of the paper S.

FIG. 17 is a diagram for explaining the contents of the fan drive control process according to the present modification.
FIG. 17A shows a state in which the leading edge of the sheet S passes through the duplex conveying roller pair 53 toward the intersection position Xb in the circulation path 58, and is detected by the sheet detection sensor SE <b> 2 on the circulation path 58. The distance from the detection position of the paper S to the intersection position Xb is indicated by L1.
Since the leading edge of the sheet S has not reached the intersection position Xb, the IU cooling fan 81 is driven, and the heated air in the IU peripheral space 19 flows in the direction indicated by the arrow G through the air blowing path 82.

  When the sheet S is transported by the distance L1 after the leading edge of the sheet S is detected by the sheet detection sensor SE2, the IU cooling fan 81 is stopped as shown in FIG. The flow is also stopped. When the trailing edge of the sheet S passes the intersection position Xb, the driving of the IU cooling fan 81 is resumed, and the cooling air flows in the air passage 82. The fact that the trailing edge of the sheet S has passed the intersection position Xb is detected by a sheet detection sensor SE4 disposed in the circulation path 58 nearer the downstream in the sheet conveyance direction than the intersection position Xb.

  As a result, while the air passage 82 is blocked by the paper S at the intersection position Xb, the IU cooling fan 81 is stopped and the backflow of the cooling air is prevented. Note that when the paper S being transported through the transport path 39 passes through the intersection position Xa of the transport path 39 and the air blowing path 82, the IU cooling fan 81 even if the air blowing path 82 is blocked by the paper S. No stop control is performed. This is because the crossing position Xa is located upstream of the IU cooling fan 81 in the direction in which the cooling air flows on the air blowing path 82, so that the air blowing path 82 is blocked by the sheet S being conveyed through the conveyance path 39 at the crossing position Xa. This is because almost no backflow occurs.

FIG. 18 is a flowchart showing the contents of the fan drive control process according to this modification. This process is executed for each sheet of paper conveyed through the circulation path 58.
As shown in the figure, the IU cooling fan 81 is driven (step S101).
It is determined whether or not the leading edge of one sheet (here, S1) being conveyed through the circulation path 58 has been detected by the sheet detection sensor SE2 (step S102).

If it is determined that the leading edge of the sheet S1 is detected by the sheet detection sensor SE2 (“YES” in step S102), it is determined whether or not a predetermined time Tx has elapsed since the leading edge of the sheet S1 was detected (step S103). The predetermined time Tx corresponds to a value obtained by dividing the distance L1 by the conveyance speed of the paper S.
If it is determined that the predetermined time Tx has elapsed (“YES” in step S103), the IU cooling fan 81 is stopped (step S104) (step S104 in FIG. 17B), assuming that the leading edge of the sheet S1 has reached the intersection position Xb. Equivalent to the state).

It is determined whether or not the trailing edge of the sheet S1 is detected by the sheet detection sensor SE4 (step S105). If it is determined that the trailing edge of the sheet S1 is detected by the sheet detection sensor SE2 (“YES” in step S105), the IU cooling fan 81 is driven assuming that the trailing edge of the sheet S has passed the intersection position Xb ( Step S106) and return.
(2) In the above-described embodiment, the configuration example of the cooling unit 8 having the air passage 82 that crosses the conveyance path 39 and the circulation path 58 has been described. However, a configuration in which a cooling unit different from this is provided may be employed.

FIG. 19A is a schematic diagram for explaining the configuration of another cooling unit 200.
As shown in the figure, the cooling unit 200 includes an IU cooling fan 201 and a blower path 202.
The blower path 202 has a lower end that is one end thereof extending to the IU peripheral space 19 so as to bypass the intermediate transfer belt 21, and an upper end that is the other end is connected to a discharge port (not shown). Crosses the discharge path 56 at Xc. The position Xc is located between the discharge roller pair 50 and the position Xd, and the position Xd corresponds to the connection position of the discharge path 56 and the circulation path 58.

When the IU cooling fan 201 is driven, the heated air in the IU peripheral space 19 is sucked using the lower end of the air passage 202 as an inlet, flows in the air passage 202 in the direction indicated by the arrow H, and the upper end that is an outlet. It is configured to be discharged out of the machine through a connected discharge port.
Thus, when the structure which further provides the ventilation path 202 which crosses the discharge path 56, when the paper S switches back by the discharge path 56, it arises that the ventilation path 202 is obstruct | occluded by the paper S, and the ventilation path 202 is formed. When blocked, the heated air in the IU peripheral space 19 cannot be discharged outside the machine via the air blowing path 202.

Therefore, in this modification, the reversal position of the paper S at the time of switchback is changed according to the temperature of the IU peripheral space 19, thereby improving the print productivity and preventing the temperature rise of the IUs 10Y to 10K. Yes.
FIG. 19B shows the direction indicated by the arrow E when the trailing edge of the paper S conveyed in the direction indicated by the arrow D reaches the third position X3 when the temperature of the IU peripheral space 19 is lower than a predetermined value. FIG. 19C is a diagram illustrating a state in which the paper is reversed when the rear end of the sheet S reaches the fourth position X4 when the temperature of the IU peripheral space 19 is equal to or higher than a predetermined value. is there.

The third position X3 is located between the position Xc and the position Xd on the discharge path 56, and the fourth position X4 is located between the position Xc and the discharge roller pair 50 on the discharge path 56. In the direction indicated by the arrow D, the positions Xd, X3, Xc, and X4 have a positional relationship arranged in this order.
If the reverse position of the sheet S is set to the third position X3 as shown in FIG. 19B, the distance between the positions Xd and X3 is shorter than the distance between the positions Xd and X4 with the position Xd as the base point. After the sheet S is reversed, the time required to guide the reversed sheet S to the circulation path 58 can be shortened, and the number of sheets conveyed per unit time can be improved and the print productivity can be improved. .

On the other hand, the air passage 202 is blocked by the paper S over the entire period from when the leading edge of the paper S passes the position Xc to when the paper S is reversed and after the trailing edge of the paper S passes the position Xc. As a result, the temperature of the IU peripheral space 19 is likely to rise.
On the other hand, if the reverse position of the sheet S is set to the fourth position X4 as shown in FIG. 19C, the trailing edge of the sheet S conveyed in the direction indicated by the arrow D passes the position Xc. Since the air passage 201 is opened after the reversal until the leading edge of the paper S (corresponding to the rear end before the reversal) reaches the position Xc, it is easy to suppress the temperature rise in the IU peripheral space 19. Become. On the other hand, since the distance between the positions Xd and X4 becomes longer, it takes longer time to guide the paper S to the circulation path 58 than when the reverse position is set to the third position X3, and the print productivity is increased. To reduce.

In this modification, in order to improve the productivity of printing as much as possible, the reference inversion position is set to the third position X3 close to the connection position Xd between the discharge path 56 and the circulation path 58, and the IU ambient temperature is a predetermined value. If it becomes above, a reverse position will be changed from the 3rd position X3 to the 4th position X4 nearer to the discharge roller pair 50 than the position Xc, and the temperature rise of IU10Y-10K will be suppressed.
20 and 21 are flowcharts showing the contents of the reverse position changing process according to this modification, and are executed for each sheet to be switched back. This process is executed by the control unit 6.

As shown in FIG. 20, the IU cooling fan 201 is driven (step S201), and the discharge roller pair 50 is rotated forward (step S202).
It is determined whether or not the leading edge of one sheet of paper S being transported on the transport path 39 has been detected by the paper discharge sensor 38 (step S203). If it is determined that the leading edge of the paper S has been detected (“YES” in step S203), the IU ambient temperature is detected (step S204). It is determined whether or not the detected temperature is lower than a predetermined value (step S205).

  If it is determined that the detected temperature is lower than the predetermined value (“YES” in step S205), the reverse position is set to the third position X3 (step S206), and the sheet stop time Tp when the sheet S is switched back is set. A predetermined first stop time, here 50 [msec], is set (step S207). This setting is performed, for example, by storing information indicating the stop time Tp in the internal storage unit, and is overwritten when information is already stored.

Then, it is determined whether or not a predetermined time Ty has elapsed since the leading edge of the paper S was detected (step S208). The predetermined time Ty is from when the leading edge of the sheet S being conveyed on the conveying path 39 passes the detection position by the sheet ejection sensor 38 until the trailing end of the sheet S guided to the ejection path 56 reaches the position X3. It corresponds to the time required for.
If it is determined that the predetermined time Ty has elapsed (“YES” in step S208), the discharge roller pair 50 is stopped (step S209). As a result, the sheet S is temporarily stopped at the third position X3 (FIG. 19B).

  On the other hand, if it is determined that the detected temperature is equal to or higher than the predetermined value (“NO” in step S205), the reverse position is set to the fourth position X4 (step S210), and the paper stop time when the paper S is switched back. Tp is set to a predetermined second stop time longer than the first stop time, here 500 [msec] (step S211). This setting method is the same as the setting method in step S207 described above. Then, it is determined whether or not a predetermined time Tz has elapsed since the leading edge of the paper S was detected (step S212).

For a predetermined time Tz, after the leading edge of the paper S being conveyed on the conveyance path 39 passes the detection position by the paper ejection sensor 38, the trailing edge of the paper S guided to the ejection path 56 reaches the fourth position X4. This corresponds to the time required to complete the process.
When it is determined that the predetermined time Tz has elapsed (“YES” in step S212), the process proceeds to step S209, and the discharge roller pair 50 is stopped. As a result, the sheet S is temporarily stopped at the fourth position X4, and the air blowing path 202 is opened (FIG. 19C).

Then, as shown in FIG. 21, the set time of the stop time Tp is read (step S213). This stop time Tp is the first stop time when the first stop time is set in step S207, and becomes the second stop time when the second stop time is set in step S211.
It is determined whether or not the read stop time Tp has elapsed since the discharge roller pair 50 was stopped (step S214). This determination is made based on whether or not the time measured by an internal timer (not shown) is started in accordance with the stop of the discharge roller pair 50 and the time measured reaches the stop time Tp. If it is determined that the stop time Tp has elapsed (“YES” in step S214), the discharge roller pair 50 is reversed (step S215), and the process ends.

As a result, when the detected temperature is lower than the predetermined value, the conveyed paper S is stopped at the third position X3 for the first stop time and then conveyed from the discharge path 56 to the circulation path 58 by switchback. When the detected temperature is equal to or higher than the predetermined value, the sheet S is stopped at the fourth position X4 for the second stop time, and then conveyed from the discharge path 56 to the circulation path 58 by switchback.
By setting the stop time Tp of the sheet S at the time of switchback to be longer than the first stop time when the detected temperature is lower than the predetermined value, the second stop time when the detected temperature is higher than the predetermined value is longer. The opening time of the path 201 can be made longer, and the cooling effect of the IUs 10Y to 10K can be enhanced.

  By adopting the configuration including the two cooling units 8 and 200 as described above, it is possible to further suppress the temperature rise of the IUs 10Y to 10K due to the heat of the paper S returned from the circulation path 58 to the image forming unit 2 during double-sided printing. It becomes possible. The above values of the first stop time and the second stop time are examples, and for example, the first stop time can be set to a shorter value, 0 seconds, or the second stop time can be set to 1 second.

In the above description, the discharge path 56 for discharging the paper S is also used as a reverse path for switching back the paper S. However, for example, when the discharge path and the reverse path are provided separately, In the inversion path, the above-described inversion position switching control is executed.
(3) In the above embodiment, the IU ambient temperature is detected, and the stop position of the paper S in the circulation path 58 is set to one of the first position X1 and the second position X2 according to the detected temperature. However, it is sufficient that the temperature of IU 10Y to 10K can be indexed, and the present invention is not limited to the configuration for detecting the IU ambient temperature. For example, the temperature of IU 10Y to 10K may be detected, and preferably the temperature of developing units 14Y to 14K may be detected. More preferably, the temperature of the IU 10K or the developing unit 14K closest to the transport path 39 may be detected.

Further, the information indicating the temperature of the IUs 10Y to 10K is not limited to the temperature detected by the temperature detection sensor 18. Instead, for example, information on the number of sheets used in a double-sided print job may be acquired.
As the number of sheets used increases, the job execution time becomes longer and the temperature of the IUs 10Y to 10K is likely to rise. For example, if the number of sheets S is smaller than a predetermined number, this is acquired as information indicating that the temperature of IU 10Y to 10K or its surroundings is lower than a predetermined value, and the stop position is set to the first position X1. When the number is greater than or equal to the predetermined number, it is acquired as information indicating that the temperature is equal to or higher than the predetermined value, and can be set to the second position X2.

Acquisition of the number of sheets can be performed by a method of receiving the number of sheets designated by the user from the operation unit 7 or the like. The predetermined number can be obtained in advance by an experiment or the like from the relationship between the number of continuous sheets to be passed and the rising temperature of IU10Y to 10K.
Furthermore, the information to be indexed may be information on the paper size (conveyance direction length). When the conveyance speed of the paper S is the same regardless of the paper size and the paper interval is the same, when the paper size increases, the time for the air passage 82 to be blocked by the paper S per unit time becomes longer, and double-sided printing is performed. This is because the temperature of IU10Y to 10K is likely to rise as the number of sheets increases. For example, when the first size paper is used, the stop position is set to the first position X1, and when the second size paper whose transport direction length is longer than the first size is used, the second position X2 is used. Can be set to It may be applied when the number of sheets is equal to or greater than a predetermined number.

For example, a paper size can be obtained by arranging a size detection sensor in the paper feed cassette 31 and receiving size detection by the sensor. The first size and the second size can be determined in advance by experiments or the like.
Further, the information to be indexed can be regarded as the driving time of the duplex conveyance motor 91 or 92. This is because the longer the drive time of the duplex conveyance motor 91 (92), the more the number of sheets conveyed through the circulation path 58 in duplex printing, and the temperature of the IUs 10Y to 10K is likely to rise. For example, within a predetermined time Tm (for example, 1 hour), the driving time is accumulated every time the double-sided conveyance motor 91 is driven, and the accumulated driving accumulated time Tj exceeds a certain time Tn (for example, 45 minutes). Until the stop position is set to the first position X1 and the predetermined time is exceeded, the stop position can be switched to the second position X2 from the next double-sided printing.

Further, the information to be indexed can be regarded as a paper type.
When the sheet S is thicker than the thin sheet, the amount of heat per unit area of the sheet S increases when the sheet S returns to the secondary transfer position 29 via the circulation path 58 after fixing. In this case, the IU ambient temperature is more likely to rise than thin paper. Therefore, the stop position is set according to the type of the sheet. Specifically, when the sheet S having the first thickness is used, the stop position of the sheet S is set to the first position X1, and the second thicker than the first is set. In the case of using a sheet S having a thickness of 2 mm, it is possible to adopt a configuration in which the second position X2 is set. It may be applied when the number of sheets is equal to or greater than a predetermined number.

Acquisition of the paper type (thick paper, thin paper, etc.) can be performed by, for example, a method of receiving paper type information from the operation unit 7 by the user.
Further, the indexing information can be a mode type of a color mode and a monochrome mode. For example, when the fixing temperature of the fixing unit 4 is different between the color mode and the monochrome mode (for example, the color is higher in temperature than the monochrome), a difference occurs in the temperature of one sheet S due to thermal fixing. This is because there is also a difference in the rising temperature of the IU peripheral space 19 due to the paper S. When the fixing temperature is higher in the color mode than in the monochrome mode, the stop position is set to the first position X1 in the monochrome mode, and is set to the second position X2 in the color mode. Can do.

When executing the double-sided print job, the paper stop position setting unit 107 acquires information such as the number of sheets in place of the temperature detected by the temperature detection sensor 18, and based on the acquired information, the stop position of the paper S is set to the first position X1 or The structure set to the 2nd position X2 can be taken.
The setting of these stop positions may be applied to the configuration of the modification (2).
(4) In the above embodiment, the paper S fed out from the paper feed cassette 31 (one on which image formation has not yet been performed) and the paper S conveyed through the circulation path 58 (image formation on the second surface completed). Both of them are directed to the secondary transfer position 29 of the image forming unit 2 via the timing roller pair 35, but are not limited thereto.

  For example, instead of the timing roller pair 35, a first timing roller pair that transports the paper S fed from the paper feed cassette 31 to the secondary transfer position 29 in accordance with the timing of image formation on the first surface of the paper S. In addition, a configuration in which a second timing roller pair that conveys the sheet S conveyed through the circulation path 58 to the secondary transfer position 29 in accordance with the timing of image formation on the second surface of the sheet S may be provided. In this configuration, the standby flag can be switched on and off in accordance with the conveyance timing of the sheet S by the first timing roller pair.

(5) In the above embodiment, it is determined using the standby flag that the paper S in the circulation path 58 needs to be temporarily stopped and waited when the leading edge of the paper S1 reaches the determination position X0. However, as long as it is possible to determine whether or not standby is necessary, the configuration is not limited to the configuration using the standby flag.
In the double-sided alternating conveyance shown in FIG. 4, for example, the first sheet S1 does not need to be stopped once, but the second and subsequent sheets S2 need to be stopped temporarily. When the number of sheets, the number of built-in sheets, and the like are determined in advance, once the sheet S reaches the determination position X0 for each sheet, it is necessary to temporarily stop the sheet S (corresponding to the standby flag being on) or Unnecessary (equivalent to turning off the standby flag) can be determined.

  (6) In the above-described embodiment, the second position X2 is a position upstream of the intersection position Xb with the blower path 82 on the circulation path 58 in the sheet conveyance direction. Therefore, it is sufficient that the air passage 82 is not completely blocked. For example, the leading edge of the sheet S may enter the intersecting position Xb, and only a part of the air passage 82 may be blocked. If there is an unobstructed part, the air flow can pass through that part.

In this case, the sheet S is temporarily stopped at a position further downstream in the transport direction, and the productivity of printing can be improved. However, the unit time that flows through the air passage 82 is that part of the air passage 82 is blocked. Since the amount of the cooling air per unit is reduced, the effect of suppressing the temperature rise of the IU is reduced. A suitable position is determined according to the device configuration.
(7) In the above embodiment, the example in which the image forming apparatus according to the present invention is applied to a tandem type printer has been described. However, the image forming apparatus is not limited to the tandem type, and regardless of the functions of color and monochrome image formation. In general, an image forming apparatus having a double-sided image forming function for forming an image such as a toner image on both sides (first side and second side) of a sheet such as paper S, for example, a copier, a facsimile machine, an MFP (Multiple Function Peripheral) ) Etc.

  Further, although the IU cooling fan 81 is disposed between the intersection position Xa with the conveyance path 9 and the intersection position Xb with the circulation path 58 on the air supply path 82, the present invention is not limited thereto. For example, you may arrange | position in the connection position with the discharge port 85 equivalent to the most downstream of the airflow in the ventilation path 82. FIG. Further, it is sufficient that the heated air in the IU peripheral space flows through the air passage 82 as cooling air, and means other than the fan may be used.

Further, the configuration example including the fixing roller 41 and the pressure roller 42 has been described. However, the configuration is not limited to this as long as the image on the sheet is thermally fixed.
In the double-sided alternate conveyance, the stop position of the paper S is switched when the number of both-sided built-in sheets is two. However, the present invention is not limited to this. It is good to do. When one sheet S reaches the determination position X0 on the circulation path 58, if the sheet S needs to be stopped once in the circulation path 58 after that, the stop position is set to the first position X1. And the second position X2, and the sheet S is stopped at the set position. Similarly, another sheet S following the sheet S is also temporarily stopped at a position downstream of the stop position.

When switching the number of double-sided built-in sheets (step S36), the number of built-in double-sided sheets is changed from M (plural) to M, even when the number of double-sided built-in sheets is 4 or more, as in the case of 3. Switching to a small number of predetermined N sheets is set. The value of N is predetermined according to the device configuration.
Furthermore, in the said embodiment, although the ventilation path 82 crossed the circulation path 58 and the conveyance path 39, it was set as the structure connected to the image formation part 2, If the ventilation path 82 can bypass the conveyance path 39 by an apparatus structure, for example In addition, a configuration that leads to the image forming unit 2 without traversing the conveyance path 39 may be adopted. In addition, by discharging the heated air in the IU peripheral space 19 through the air passage 82, the temperature rise of the IUs 10Y to 10K is prevented (IU 10Y to 10K is cooled). For example, outside air is sent from the air passage 82 to the IU peripheral space. It is also possible to cool the IUs 10Y to 10K by sending them to the unit 19.

Furthermore, the present invention is not limited to duplex conveyance, and switching of the stop position of the sheet S may be applied even in duplex circulation conveyance, for example.
Further, the above embodiment and the above modification examples may be combined.

  The present invention can be widely applied to image forming apparatuses having a function of forming images on both sides of a sheet.

2 Image forming unit 3 Feeding unit 4 Fixing unit 5 Double-sided transport unit 6 Control unit 8,200 Cooling unit 10Y, 10M, 10C, 10K Image forming unit (IU)
14Y, 14M, 14C, 14K Developing unit 18 Temperature detection sensor 19 IU peripheral space 21 Intermediate transfer belt 35 Timing roller pair 39 Conveying path 50 Discharging roller pair 56 Discharging path (reversing path)
58 Circulating path 81, 201 IU cooling fan 82, 202 Air path 102 CPU
107 sheet stop position setting unit 108 timing processing unit 109 sheet conveyance stop control unit 110 fan drive control unit S sheet X0 determination position X1 first position X2 second position X3 third position X4 fourth position Xa between the conveyance path and the air supply path Intersection position Xb Intersection position between circulation path and air passage Xc Intersection position between discharge path (reversal path) and air path Xd Connection position of exhaust path (reversal path) and circulation path

Claims (13)

An image is formed on the first surface of the conveyed sheet by the image forming unit, the image is heat-fixed on the sheet, the sheet is transported through the circulation path, and returned to the image forming unit. An image forming apparatus for forming an image, thermally fixing the image to a sheet, and then discharging the sheet,
Acquisition means for acquiring information indicating the temperature of the image forming unit or its surroundings;
A cooling means for cooling the image forming section through cooling air through the air passage passing through the circulation path and leading to the image forming section;
A conveyance stop means for temporarily stopping and waiting in the circulation path in order to take a timing to send out the sheet being conveyed through the circulation path toward the image forming unit;
A cooling means different from the cooling means;
Switchback means for reversing the sheet in the reversing path after thermally fixing the sheet on which the image is formed on the first surface ,
The transport stop means is
If the indexing information indicates that the temperature is lower than a predetermined value, the sheet is moved to a first position where the leading end of the sheet is located downstream in the sheet conveying direction from the position where the air passage crosses the circulation path. If it is stopped and shows a predetermined value or more, it is stopped at the second position upstream of the first position in the sheet conveying direction and the sheet does not block the air passage ,
Said another cooling means is:
Crossing the reversing path and leading to the image forming section, having an air blowing path different from the air blowing path, cooling the image forming section through cooling air through the other air blowing path,
The switchback means includes
If the index information indicates that the temperature is lower than a predetermined value, the third position where the sheet trailing edge is located upstream in the sheet conveying direction from the intersection position where the other air passage crosses the reversing path. The sheet is reversed at a fourth position that is downstream of the third position in the sheet conveying direction and does not block the other air passage if the sheet exhibits a predetermined value or more. Image forming apparatus. The second position is
The image forming apparatus according to claim 1, wherein the leading end of the sheet is located upstream of the crossing position in the sheet conveying direction. The second position is
The image forming apparatus according to claim 1, wherein the leading end of the sheet is a position that blocks only a part of the air passage. The cooling means is
A fan for blowing cooling air to the air passage;
4. The air blow by the fan is performed when the sheet is stopped at the second position, and the air blow is stopped when the sheet is stopped at the first position. The image forming apparatus described in the item. The cooling means is
5. The image forming apparatus according to claim 4, wherein the air blowing by the fan is stopped while the air blowing path is blocked by a sheet conveyed through the circulation path. The switchback means includes
When reversing the sheet conveyed on the reversing path at the third position, the reversing is performed after stopping at the third position for the first stop time, and when reversing at the fourth position, 6. The image forming apparatus according to claim 1, wherein reversal is performed after stopping at the fourth position for a second stop time longer than the first stop time. 6. The inversion path is
7. The apparatus according to claim 1, wherein the sheet is provided between a position where the sheet after image formation on the first surface is thermally fixed and the circulation path. The image forming apparatus described. Comprising temperature detecting means for detecting the temperature;
The acquisition means includes
As the information, the detection result of the temperature detection means is acquired,
The transport stop means is
The sheet is stopped at a first position when a temperature detected by the temperature detection unit is lower than a predetermined value, and the sheet is stopped at a second position when the temperature is equal to or higher than a predetermined value. Item 8. The image forming apparatus according to any one of Items 1 to 7 . The acquisition means includes
As the information, obtaining information indicating the number of sheets in a job for forming an image on a plurality of sheets conveyed continuously,
The transport stop means is
When the number of sheets is less than a predetermined number, the temperature is lower than a predetermined value, and the sheet is stopped at the first position. When the number is more than a predetermined number, the temperature is a predetermined value. as indicating the above, the image forming apparatus according to any one of claims 1 to 7, characterized in that stops the sheet to the second position. The acquisition means includes
As the information, information indicating the size of the conveyed sheet is acquired,
The transport stop means is
When the sheet size is the first size, the sheet is stopped at the first position to indicate that the temperature is lower than a predetermined value, and the second size is longer in the conveyance direction than the first size. in the case of the assumption that the temperature indicates a predetermined value or more, the image forming apparatus according to any one of claims 1 to 7, characterized in that stops the sheet to the second position. The acquisition means includes
As the information, obtaining information indicating the thickness of the conveyed sheet,
The transport stop means is
When the thickness of the sheet is the first thickness, the sheet is stopped at the first position to indicate that the temperature is lower than a predetermined value, and the second thickness is larger than the first thickness. in this case, assuming that the temperature indicates a predetermined value or more, the image forming apparatus according to any one of claims 1 to 7, characterized in that stops the sheet to the second position. A pair of conveying rollers provided in the circulation path for conveying the sheet;
A conveyance motor that rotationally drives the conveyance roller pair,
The acquisition means includes
As the information, obtaining information indicating the accumulated driving time of the transport motor,
The transport stop means is
When the drive integration time does not exceed a certain time within a predetermined time, the sheet is stopped at the first position to indicate that the temperature is lower than a predetermined value, and when the certain time is exceeded. They include, but are said temperature indicates a predetermined value or more, the image forming apparatus according to any one of claims 1 to 7, characterized in that stops the sheet to the second position. Image forming one sheet at a time while accommodating a maximum of M (plural) sheets on the path until the sheet returns to the image forming section after image formation on the first surface. Including job execution means for executing a job for performing image formation on the second surface.
The job execution means includes:
When the temperature becomes equal to or higher than a predetermined value during job execution, the number of sheets accommodated on the path is switched to a predetermined N number less than M.
The transport stop means is
Wherein the number of sheets is switched to N frames contained in the path, in any one of claims 1 to 12, characterized in that the stop against sheet after switching to the second position Image forming apparatus.

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