JP4973088B2 - 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 (front surface) and a second surface (back surface) of a sheet.

  In an image forming apparatus such as a copying machine, for example, M (M is an integer of 2 or more) sheets are conveyed one by one to a transfer position, and a toner image is transferred to the surface of the sheet. While performing image formation in order to fix the image on the sheet by heating, the sheet after image formation on the surface is reversed and conveyed one by one through the circulation conveyance path, and image formation on the surface of all M sheets After the completion, each sheet conveyed through the circulation conveyance path is conveyed again to the transfer position to transfer the toner image to the back surface of the sheet, and the transferred toner image is fixed on the sheet by heating and then discharged. Some have a so-called circulation type double-sided printing function that repeatedly executes a series of operations of storing in a tray in units of M sheets.

When such a circulation system is used, the number of sheets on which images can be formed per unit time can be improved. However, tacking peculiar to double-sided printing in which sheets stored in a storage bin stick to each other may occur. . This is due to the following reason.
That is, the sheet immediately after fixing is in a high temperature state due to the heat of fixing, and if the sheet having such a high temperature continues to be stored on the tray so as to be stacked in a large amount vertically, the stored sheet Due to the heat accumulation of the bundle, the toner image fixed on the back surface of the upper sheet and the toner image fixed on the surface of the lower sheet of the two overlapping sheets come into contact with each other in a softened state and are attracted. Because.

If the sheets stuck by tucking are forcibly separated from each other, the toner image is peeled off, and it is necessary to perform printing again. Tacking occurs in the state where the sheet temperature is high as described above.
As an apparatus that cools a sheet after fixing, for example, Patent Document 1 discloses an image forming apparatus that includes a cooling apparatus and that cools and discharges a cooling gas to a high-temperature sheet after heat fixing. .
JP-A-10-90965

However, the image forming apparatus disclosed in Patent Document 1 needs to include a cooling device, which increases the cost.
SUMMARY An advantage of some aspects of the invention is that it provides an image forming apparatus capable of preventing occurrence of tacking without a cooling device.

In order to achieve the above object, an image forming apparatus according to the present invention conveys M sheets (M is an integer of 2 or more) one sheet at a time to an image forming position, and uses toner on the first surface of the sheet. While performing image formation sequentially, the toner image formed on the first surface is thermally fixed, and then the sheet is reversed and conveyed one by one through the circulation conveyance path, and image formation on the first surface of all M sheets is performed. After the completion, each sheet conveyed through the circulation conveyance path is conveyed again to the image forming position, and image formation with toner on the second surface of the sheet is sequentially performed, and the toner image formed on the second surface is obtained. A first duplex mode in which an image forming operation for discharging the sheet after thermal fixing is repeated in units of M sheets, and an image forming operation in which the value of M is changed to a value of 1 or more (M−1) or less. Image formation that can be performed by switching between two duplex modes And stage, N sheets (N is an integer greater than M) in the case of the sheet to the image formation, P th first picture (P is a predetermined value smaller than the larger N than M Up to an integer), the duplex mode is set to the first duplex mode, and the image forming unit performs the image forming operation in the first duplex mode until the Pth sheet, and the (P + 1) th and subsequent sheets The control unit for switching to the second double-side mode to execute the image forming operation, the storage unit for storing the discharged sheet, and the detection for detecting the removal of the sheet stored in the storage unit And when the detection unit detects that the sheet stored in the storage unit has been removed after switching to the second double-side mode, the control unit is configured to perform subsequent image formation. The first operation Characterized in that to execute in duplex mode.

According to the above configuration, the number of sheets on which image formation is possible per unit time can be improved by executing the first duplex mode, and the speed of the image forming operation can be increased. In the second duplex mode, compared to the first duplex mode. The number of images formed per unit time can be reduced to widen the sheet interval, and in the case of thermal fixing, the heat accumulated in the discharged sheet bundle is more easily radiated than in the first double-side mode, and the sheet bundle It is possible to suppress the temperature of the toner from rising to the melting temperature of an image such as a toner image, and to increase the speed of the next image forming operation while preventing the occurrence of tacking.

Also, the image forming apparatus according to the present invention, M sheets (M is an integer of 2 or more) sheets of conveyed to the image forming position, one by one, successively image formation by the toner on the first surface of the sheet The toner image formed on the first surface is thermally fixed, the sheet is reversed and conveyed one by one through the circulation conveyance path, and after the image formation on the first surface of all M sheets is completed, After each sheet conveyed through the circulation conveyance path is conveyed again to the image forming position, image formation with toner on the second surface of the sheet is sequentially performed, and the toner image formed on the second surface is thermally fixed. A first double-side mode in which the image forming operation for discharging the sheet is repeated in units of M sheets, and a second double-side mode in which the value of M is changed to a value not less than 1 and not more than (M−1). Image forming means that can be executed by switching between N and N sheets (N is When image formation is performed on sheets of an integer greater than M), the duplex mode is set for the first sheet to the Pth sheet (P is a predetermined value, an integer larger than M and smaller than N). The first double-side mode is set, and the image forming operation in the first double-side mode is executed by the image forming unit until the P-th sheet, and the (P + 1) th and subsequent sheets are switched to the second double-side mode. Post-processing that includes a control unit that switches to execute an image forming operation and a plurality of storage bins including first and second storage bins, and that can store a discharged sheet by switching to one of the plurality of storage bins And the control means switches to the second duplex mode and then switches the storage destination of the discharged sheet from the first storage bin to the second storage bin. The image forming operation is It is performed in one of the duplex mode, characterized by a sheet after image formation be accommodated in the second accommodating bottle according to a first duplex mode.

With this configuration, in the configuration including the post-processing unit including a plurality of storage bins, it is possible to increase the speed of the next image forming operation while preventing the occurrence of tacking.
Further, the image forming apparatus according to the present invention conveys M sheets (M is an integer of 2 or more) one by one to the image forming position, and sequentially performs image formation with toner on the first surface of the sheet. On the other hand, after the toner image formed on the first surface is thermally fixed, the sheet is reversed and conveyed one by one through the circulation conveyance path. After the image formation on the first surface of all M sheets is completed, the circulation is performed. Each sheet transported on the transport path is transported again to the image forming position, and image formation with toner on the second surface of the sheet is sequentially performed. After the toner image formed on the second surface is thermally fixed, the sheet A first double-sided mode in which an image forming operation for discharging a sheet is repeated in units of M sheets, and a second double-sided mode in which the value of M is changed to a value not less than 1 and not more than (M−1) and an image forming operation is performed. Image forming means that can be executed by switching between N sheets (N , When an image is formed on a sheet of an integer greater than M), the duplex mode is used for the first sheet to the P sheet (P is a predetermined value and is an integer larger than M and smaller than N). Is set to the first double-sided mode, and the image forming unit performs the image forming operation in the first double-sided mode until the P-th sheet, and the second double-side mode is performed for the (P + 1) th and subsequent sheets. And a plurality of storage bins including first and second storage bins, and a discharged sheet can be stored by switching to one of the plurality of storage bins. A processing unit, wherein the control means makes the value of P different between when the sheet is stored in the first storage bin and when the sheet is stored in the second storage bin. To do.

In this way, in a configuration including a post-processing unit including a plurality of storage bins, it is possible to execute a process for speeding up the image forming operation while preventing the occurrence of tacking for each storage bin.
The control unit includes a controller unit that instructs an image forming operation in the double-side mode, and an engine control unit that controls the image forming operation of the image forming unit in response to an instruction from the controller unit. It is characterized by.

  In this way, if the controller unit and the engine control unit are configured to share functions between the instruction issuing side and the instruction receiving side, for example, when a design change occurs, one firmware of the controller unit and the engine control unit It is possible to simplify the work required for the change, such as changing only one, and it is also possible to replace only one member when replacement is necessary, and the entire control means is completely replaced because the function is not shared This is advantageous in terms of cost compared to the case where it is necessary to do so.

Hereinafter, an embodiment of an image forming apparatus according to the present invention will be described by taking a tandem color digital printer (hereinafter simply referred to as “printer”) as an example.
(First embodiment)
FIG. 1 is a diagram illustrating an overall configuration of a printer 1 according to the present embodiment.
As shown in the figure, the printer 1 has a circulation type duplex printing function, and includes an image process unit 10, a feeding unit 20, a fixing unit 30, a duplex conveying unit 40, a sorter 50, a control unit 60, and the like. When an execution instruction for a print (print) job is received from an external terminal device (not shown) connected to a network, here, a LAN, the print job is executed based on the instruction. The print job includes a job in a single-side mode for forming an image on one side of a sheet and a job in a double-side mode for forming an image on the first side (front side) and the second side (back side) of a sheet.

The image processing unit 10 includes image forming units 2Y, 2M, 2C, and 2K corresponding to each of yellow (Y), magenta (M), cyan (C), and black (K), an intermediate transfer belt 11, and the like. Prepare.
The image forming unit 2Y includes a photosensitive drum 3, a charging unit 4, an exposure unit 5, a developing unit 6, a primary transfer roller 7, and a cleaner 8 that are rotationally driven in the direction of arrow A. The other image forming units 2M to 2K have the same configuration, and the reference numerals are omitted in the figure.

The intermediate transfer belt 11 is stretched around a driving roller 12, a driven roller 13, and a tension roller 14, and is driven to rotate in the direction of arrow B.
The feeding unit 20 includes a sheet feeding cassette 21 that accommodates the sheet S, a feeding roller 22 that feeds the sheets S in the sheet feeding cassette 21 one by one toward the conveyance path 27, and a conveyance that conveys the fed sheet S. A roller pair 23, a timing roller pair 24 for timing to send the sheet S to a secondary transfer position 15 as an image forming position, a secondary transfer roller 25, and the like are provided.

The fixing unit 30 includes a fixing heater 31, and the controller 60 controls the energization of the fixing heater 31 to be maintained at a fixing temperature, for example, 180 ° C.
The double-sided conveyance unit 40 includes a reversing roller pair 41 and double-sided conveyance roller pairs 42, 43, and 44. The sheet after image formation on the first surface is completed in order to form an image on the second surface in the duplex mode. The sheet S is reversed and conveyed again to the secondary transfer position 15 via the circulation conveyance path 45.

The sorter 50 includes a first storage bin 51, a second storage bin 52, discharge roller pairs 53 and 54, and stores the discharged sheet S in any of the storage bins.
The control unit 60 receives an image signal transmitted from an external terminal device, converts this to a digital image signal for Y to K colors, and controls the image processing unit 10, the feeding unit 20, and the like. The print operation is executed.

  Specifically, in the single-sided mode, after the photosensitive drum 3 rotating in the direction of arrow A is cleaned by the cleaner 8 for each of the image forming units 2Y to 2K, it is uniformly charged by the charging unit 4, The surface of the charged photosensitive drum 3 is exposed from the exposure unit 5 to form an electrostatic latent image. The formed electrostatic latent image is developed with toner as a developer by the developing unit 6 to be visualized as a toner image. The developed color toner images are primarily transferred from the photosensitive drum 3 onto the intermediate transfer belt 11 by the action of electrostatic force by the primary transfer rollers 7. 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 11.

Each color toner image on the intermediate transfer belt 11 is moved to the secondary transfer position 15 by the rotation of the intermediate transfer belt 11.
On the other hand, the sheet S is fed from the feeding unit 20 via the timing roller pair 24 in accordance with the movement timing of each color toner image on the intermediate transfer belt 11, and the sheet S rotates. The intermediate transfer belt 11 and the secondary transfer roller 25 are nipped and conveyed. At the secondary transfer position 15, the toner images of the respective colors on the intermediate transfer belt 11 are collectively applied to the first surface of the sheet S by electrostatic force. Transcribed. The surface of the intermediate transfer belt 11 after the secondary transfer is cleaned by the cleaner 16.

  The sheet S that has passed the secondary transfer position 15 is conveyed to the fixing unit 30, where the toner image is heated and pressed to be fixed on the sheet S, and then sent to the sorter 50 via the reverse roller pair 41. . The sorter 50 stores the sheet S in either the first storage bin 51 or the second storage bin 52 according to an instruction from the control unit 60. Specifically, when the first storage bin 51 is instructed, the sheet S is transported on the transport path 57 and stored in the first storage bin 51 via the transport roller pair 53. When the second storage bin 52 is instructed, the transport path is switched, transported on the transport path 58, and stored in the second storage bin 52 via the transport roller pair 54.

In the duplex mode, the reversing roller pair 41 and the conveying roller pair 53 are driven in reverse at the timing just before the trailing edge of the sheet S on which the image is formed on the first surface passes the reversing roller pair 41. As a result, the sheet S is reversed, conveyed in the direction of arrow C, and guided to the circulation conveyance path 45.
The sheet S guided to the circulation conveyance path 45 is returned again to the conveyance path 27 by the duplex conveyance roller pairs 42 to 44, and is conveyed again to the secondary transfer position 15 through the timing roller pair 24. In synchronization with the conveying operation, primary transfer of each color toner image to the second surface is performed in the image processing unit 10, and each color toner image superimposed on the intermediate transfer belt 11 is collectively collected at the secondary transfer position 15. Secondary transfer is performed on the second surface of the sheet S. The sheet S on which the color toner images are secondarily transferred onto the second surface is sent to the sorter 50 via the fixing unit 30 and the reverse roller pair 41 and is stored in the first storage bin 51 or the second storage bin 52.

  In the duplex mode, as a circulation type duplex printing function, M (M is an integer of 2 or more) sheets S is used as one unit, and image formation can be repeatedly performed for each unit. Specifically, for example, when M = 2, the first sheet S and the second sheet S are sequentially fed out from the sheet feeding cassette 21 and placed on the first surface of each sheet S in the order of the first sheet and the second sheet. After the image formation, the image is formed again on the second surface in the order of the first and second sheets after returning to the conveyance path 27 again through the circulation conveyance path 45, and then discharged, and then the third sheet, The operation of feeding out the fourth sheet S from the sheet feeding cassette 21 and performing image formation in the order of the first side and the second side of each sheet S is repeated in units of two sheets.

  In this way, when a method is used in which a plurality of sheets are taken as one unit, the preceding sheet S (the sheet S on which the image has been formed on the first surface) is circulated and conveyed while image formation on the first surface is being performed. 45 are successively conveyed, so that one sheet is used as a unit, that is, an image is formed on the first surface of the first sheet S, and the first sheet S is transferred from the circulation conveyance path 45 to the conveyance path 27. Returning is a method in which the image formation is performed on the second surface of the first sheet S and discharged, and then the second sheet S is fed to repeat the operation of forming the image on the first surface and the second surface. However, the waiting time for image formation from image formation on the first surface to image formation on the second surface is reduced, and the number of printable sheets per unit time (hereinafter referred to as “productivity”) is increased. Can do.

In the present embodiment, M = 2 is basically set in the duplex mode, but M = 1 is set when a predetermined condition for preventing the occurrence of tacking is satisfied as will be described later. It has become.
The paper feed motor 71 rotates the feed roller 22 via a drive mechanism (not shown). The drive motor 72 rotationally drives the photosensitive drum 3, the intermediate transfer belt 11, the timing roller pair 24, the secondary transfer roller 25, and the like, and the double-sided conveyance motor 73 drives the reverse roller pair 41 and the double-sided conveyance roller pairs 42 to 44. Drive to rotate.

  The sorter 50 is provided with sheet detection sensors 55 and 56. The sheet detection sensors 55 and 56 are known reflection-type optical sensors including a light emitting element and a light receiving element, and detect whether or not the sheet S is accommodated. Specifically, the sheet detection sensor 55 reflects the light emitted from the light emitting element on the sheet S accommodated in the first accommodation bin 51, and the sheet S accommodates when the reflected light is received by the light receiving element. If the reflected light is not received, an electric signal indicating that the sheet S is not accommodated is sent to the control unit 60. The same applies to the sheet detection sensor 56.

FIG. 2 is a diagram illustrating a configuration of the control unit 60.
As shown in the figure, the control unit 60 is roughly divided into an engine control unit 61 and a controller unit 62, and the engine control unit 61 and the controller unit 62 can exchange data such as signals with each other.
The controller unit 62 includes a CPU 621, a ROM 622, a memory 623, and an interface (I / F) unit 624, and prints an instruction signal for an image forming operation to the engine control unit 61, for example, which image on which side of what sheet. More specifically, a signal instructing execution of printing on the first side of the first sheet in the duplex mode, a signal instructing execution of printing on the first side of the second sheet, etc. Output at appropriate timing.

  The engine control unit 61 includes a CPU 611, a ROM 612, and a memory 613. Upon receiving an instruction from the controller unit 62, the engine control unit 61 controls the image processing unit 10, the feeding unit 20, and the like to execute the instructed image forming operation. For example, when a signal instructing execution of printing on the first side of the first sheet is received, the first sheet S is fed out from the paper feed cassette 21 to perform printing on the first side of the sheet S. In the mode, the sheet S is returned to the conveyance path 27 via the circulation conveyance path 45, and an operation of printing an image on the second surface is executed.

Further, during the image forming operation, for example, the rotational speeds of various motors such as the paper feed motor 71 are controlled so that the rotational speed of the photosensitive drum 3 and the conveying speed of the sheet S become predetermined speeds. The operation of the members related to image formation is directly controlled, such as lighting control of the fixing heater 31 based on a detection signal of a temperature sensor (not shown) so that the temperature of the toner becomes the fixing temperature.
In this sense, it can be said that the control unit 60 is divided into an engine control unit 61 that is instructed to perform an image forming operation and a controller unit 62 that is instructed to perform an image forming operation. According to this configuration, the controller unit 62 side has a control program for instructing an image forming operation, for example, a control necessary for switching the duplex unit number from two to one and returning from one to two. A program for controlling the paper feed motor 71 and the like according to the instruction can be stored on the engine control unit 61 side. The division of roles of (firmware) is distinguished and the design becomes easy.

In addition, when it is necessary to change the paper feed timing or the like, it is only necessary to change the firmware of the controller unit 62, and the design can be easily changed. Further, for example, when the control unit is replaced for the change, only the controller unit 62 needs to be replaced, which is advantageous in terms of cost.
The ROM 612 of the engine control unit 61 stores a program for controlling the image processing unit 10 and the like. The CPU 611 reads the program from the ROM 612 and controls the image forming operation. The memory 613 serves as a work area when the CPU 611 executes a program.

  On the other hand, the ROM 622 of the controller unit 62 stores a program for instructing the engine control unit 61 to perform an image forming operation. The CPU 621 reads out the program from the ROM 622 and outputs an image forming operation instruction signal to the engine control unit 61. The memory 623 becomes a work area when the CPU 621 executes a program. The interface unit 624 is an interface for connecting to a LAN (network) such as a LAN card or a LAN board.

FIG. 3 is a diagram illustrating an example of an experimental result indicating whether or not tacking has occurred when printing in the duplex mode is performed using the printer 1 under the conditions of the patterns A to D. Here, it is assumed that the discharged sheets S are sequentially stored so as to be stacked on the first storage bin 51 as the sheet storage portion.
As shown in the figure, the pattern A is an example of a job in which double-sided printing is continuously performed on 100 sheets S in the double-side mode in units of two (M = 2), and occurrence of tacking was observed. .

Pattern B is an example of a job in which double-sided printing is continuously performed on 100 sheets S in the single-sided (M = 1) double-sided mode. In this example, no tacking occurred.
In the case of the pattern A, since the unit of two sheets is used, the productivity can be increased, but the number of sheets stored per unit time in the first storage bin 51 is increased accordingly. The sheets S stored in the first storage bin 51 are immediately after passing through the fixing unit 30 and are in a high temperature state, for example, about 60 ° C., and a large number of such high-temperature sheets S are in a short time. If they are stacked, the heat of the sheet bundle will not be in time, and heat will be stored, and the temperature of the sheet bundle will rise, and it seems that tacking has occurred.

  On the other hand, in the case of the pattern B, the number of sheets accommodated per unit time in the first accommodation bin 51 is smaller than that of the unit of two sheets because of the unit of one sheet. Specifically, the time (sheet interval) from when the nth sheet S is stored until the next (n + 1) th sheet is stored becomes longer. Therefore, it is considered that the heat of the sheet bundle is released through the first storage bin 51 and the like until the next sheet is accommodated, and the temperature of the sheet bundle is lowered.

  For this reason, in order to prevent the occurrence of tacking while ensuring productivity to some extent, double-sided printing is repeated in the duplex mode in units of two at the start of the job, and the number of printed sheets P (the relevant P sheets) in the middle of the job After that, if double-sided printing in units of two sheets is continued, the temperature of the sheet bundle is assumed to rise to the temperature at which tacking will occur. Thus, it can be considered not to raise the temperature of the sheet bundle.

In the printer 1, when the number of sheets P is 80 as shown in the pattern C, it was confirmed that no tacking was observed.
Further, as shown in the pattern D, after the job by the pattern C is finished and the sheet bundle is stored in the first storage bin 51, after a certain time T, in the example of FIG. It was confirmed that tacking does not occur even if the next job is started in double-sided mode in units of two sheets and sheets S are accommodated so as to be further stacked on the accommodated sheet bundle. This is considered to be because the temperature of the sheet bundle was sufficiently lowered by heat dissipation in 10 minutes.

Therefore, if the number P and the time T are obtained in advance from experiments or the like as threshold values for switching the unit number M during duplex printing, the occurrence of tacking can be prevented while securing a certain degree of productivity. .
FIG. 4 is a diagram illustrating the contents of a flowchart in the control processing when the controller unit 62 executes a print job in the duplex mode.

  As shown in the figure, when it is detected that power is supplied to the printer 1 (step S1), the duplex unit sheet number is set to “2” and the number of printed pages and the stop time are reset to “0” (step S2). ). Here, the duplex unit number is the value M. The number of printed surfaces is the total number of the front and back surfaces of the sheet S by double-sided printing. The stop time corresponds to a time when the printer 1 is not executing a print job, specifically, a standby time after the end of one print job until the start of the next print job. Data indicating the number of duplex units, the number of prints, and the stop time is stored in a non-volatile memory (not shown) in the control unit 60, and is read and set or updated as necessary.

When a print job to be executed is received (“YES” in step S3), the received print job is registered in a management table (not shown) (step S4). Here, information such as the job management number, the number of prints, and the size of the sheet to be used are registered.
Then, it is determined whether or not the current number of printed faces matches the value indicating the final face (step S5). Here, the final surface refers to the final surface when printing on each surface in order from the first surface for one print job. For example, in the case of the job indicated by the pattern C, the number of prints is 160, and each sheet is printed on the front and back (2 sides). Therefore, the final surface is the 320th surface, and this value “320” is the final surface. It becomes the value shown. Hereinafter, the case of the pattern C will be described as an example.

If it is determined that the number of printed surfaces does not match the value indicating the final surface (“NO” in step S5), “1” is incremented to the current number of printed surfaces “0” and the stop time is reset to “0”. (Step S6). Here, the number of print planes = 1.
It is determined whether or not the current number of printed faces> the predetermined number of faces (step S7). Here, the predetermined number of faces is a value obtained by multiplying the number P at the time of switching by “2”. As described above, in the printer 1, since the predetermined number P = 80, the predetermined number of faces = 160.

Here, since the current number of printed faces = 1, it is determined that the number of printed faces> the predetermined number of faces is not satisfied (“NO” in step S7), and the process proceeds to step S9.
In step S9, the engine control unit 61 is instructed to print the first sheet S on the first surface, and the process returns to step S5.
FIGS. 5A and 5B are timing charts when the controller unit 62 instructs the engine control unit 61 to perform printing. FIG. 5A illustrates a case where the duplex unit number M = 2, and FIG. = 1.

  As shown in FIG. 5A, when the engine control unit 61 receives a print instruction (print instruction signal 71) on the first surface of the first sheet S from the controller unit 62, the paper feed cassette 21 The first sheet S is fed out, printing is performed on the first surface of the sheet S, and an operation of leading to the circulation conveyance path 45 is executed. When the printing is normally performed, the engine control unit 61 sends a response signal 72 indicating that to the controller unit 62. For example, when a trouble such as a paper jam is detected, the response signal is not transmitted.

  Returning to FIG. 4, it is determined in step S5 whether or not the number of print faces matches the value indicating the final face. Since the current number of printed faces = 1, it is determined that they do not match (“NO” in step S5), and “1” is incremented to the current number of printed faces = 1 (step S6). Here, the number of print surfaces = 2. The stop time is also reset, but the description is omitted below.

  Since the number of printed faces = 2, it is determined in step S7 that the number of printed faces> the predetermined number of faces is not satisfied, and the process proceeds to step S9. In step S9, since the current duplex unit number = 2, the engine control unit 61 is instructed to print the first sheet S on the first side (signal 73 in FIG. 5A), and the process proceeds to step S5. Return. This instruction is performed only when the response signal 72 from the engine control unit 61 is received.

When the engine control unit 61 receives the print instruction signal 73 from the controller unit 62, the engine control unit 61 feeds the second sheet S from the sheet feeding cassette 21 and prints on the first surface of the sheet S to print the first sheet. It leads to the circulation conveyance path 45 so that S may be followed. When the operation is normally performed, a response signal 74 is sent to the controller unit 62.
In step S5, it is determined that the current print surface number = 2, so that it does not coincide with the final surface. In step S6, "1" is incremented to the current print surface number = 2. In step S7, since the number of printed faces = 3, it is determined that the number of printed faces> the predetermined number of faces is not satisfied, and the process proceeds to step S9. In step S9, since the current duplex unit number = 2, the engine control unit 61 is instructed to print the second sheet of the first sheet S (signal 75 in FIG. 5A), and the process proceeds to step S5. Return.

Upon receiving the print instruction signal 75, the engine control unit 61 returns the first sheet S conveyed through the circulation conveyance path 45 to the conveyance path 27 again, performs printing on the second surface, and then discharges it. Perform the action. When the operation is normally performed, a response signal 76 is sent to the controller unit 62.
Again, in step S5, it is determined that the current print face number = 3, so that it does not coincide with the final face. In step S6, “1” is incremented to the current print face number = 3. In step S7, since the number of printed faces = 4, it is determined that the number of printed faces> the predetermined number of faces, and the process proceeds to step S9. In step S9, since the current duplex unit number = 2, the engine control unit 61 is instructed to print the second sheet S on the second side (signal 77 in FIG. 5A), and the process proceeds to step S5. Return.

Upon receiving the print instruction signal 77, the engine control unit 61 returns the second sheet S conveyed through the circulation conveyance path 45 to the conveyance path 27 again, performs printing on the second surface, and then discharges it. Perform the action. When the operation is normally performed, a response signal 78 is sent to the controller unit 62.
In step S5 again, it is determined that the current print surface number = 4, so that it does not coincide with the final surface. In step S6, “1” is incremented to the current print surface number = 4. In step S7, since the number of printed faces = 5, it is determined that the number of printed faces> the predetermined number of faces is not satisfied, and the process proceeds to step S9. In step S9, since the current duplex unit number = 2, the engine control unit 61 is instructed to print the first sheet S on the first side (signal 79 in FIG. 5A), and the process proceeds to step S5. Return.

When the engine control unit 61 receives the print instruction signal 79, the engine control unit 61 performs the operation of feeding the third sheet S from the sheet feeding cassette 21, printing on the first surface of the sheet S, and guiding it to the circulation conveyance path 45. .
Thereafter, the same operation as that for the first and second sheets S is repeated until the third, fourth, fifth, sixth,..., 79th and 80th sheets. Print.

In step S9, an instruction to print on the second surface of the 80th sheet S is given. In step S6, "1" is incremented to the current number of printed pages = 160. As a result, the number of print surfaces = 161.
In step S7, it is determined that the number of printed faces> the predetermined number of faces (= 160), and the process proceeds to step S8. In step S8, the duplex unit sheet number is set to 1, and the process proceeds to step S9.

In step S9, since the current duplex unit number = 1, the engine controller 61 is instructed to print the first sheet of the 81st sheet S (signal 81 in FIG. 5B), and step S5. Return to.
When the engine control unit 61 receives the print instruction signal 81, the engine control unit 61 feeds the 81st sheet S from the sheet feeding cassette 21, prints the first surface of the sheet S, and guides it to the circulation conveyance path 45. When the operation is normally performed, a response signal 82 is sent to the controller unit 62.

  In step S5, since the current number of printed faces = 161, it is determined that it does not coincide with the final face. In step S6, “1” is incremented to the current number of printed faces 161. In step S7, since the number of printed faces = 162, it is determined that the number of printed faces> the predetermined number of faces, and the process proceeds to step S8. In step S8, the duplex unit sheet number is set to 1, and the process proceeds to step S9.

In step S9, since the current duplex unit number = 1, the engine control unit 61 is instructed to print the second sheet of the 81st sheet S (signal 83 in FIG. 5B), and step S5. Return to.
When the engine control unit 61 receives the print instruction signal 83, the engine control unit 61 returns the 81st sheet S transported through the circulation transport path 45 to the transport path 27 again, and after printing on the second surface, discharges it. Perform the action. When the operation is normally performed, a response signal 84 is sent to the controller unit 62.

Thereafter, the same operation is repeated up to the 82nd sheet, the 83rd sheet, and the 160th sheet, and duplex printing is performed in the duplex mode for each sheet. In this sense, it can be said that the image processing unit 10, the feeding unit 20, the duplex conveyance unit 40, and the like function as image forming units that can be executed by switching between the M = 2 duplex mode and the M = 1 duplex mode. .
After instructing printing on the second side of the 160th sheet S in step S9, when returning to step S5, since the current number of printed sides = 320 at that time, it is determined that it matches the final side, Control goes to step S10. The fact that the number of print faces matches the final face means the end of the print job.

In step S10, the stop time is counted. Specifically, when the current stop time = 0, the stop time is counted after the response signal indicating that the printing operation for the final surface has been normally performed is received from the engine control unit 61. . Further, if the count is currently being performed, the count is continued.
It is determined whether or not stop time> predetermined time (step S11). Here, the predetermined time is 10 minutes. This is a time corresponding to 10 minutes in the pattern D.

If it is determined that stop time> predetermined time (“YES” in step S11), the unit number of duplex units is reset to 2 and the number of printed pages is reset to “0” (step S12), and the process returns to step S3. .
On the other hand, if it is determined that stop time> predetermined time is not satisfied, that is, stop time ≦ predetermined time (“NO” in step S11), the process returns to step S3.

In step S3, if the next print job is not accepted, the process returns to step S10, and the stop time is continuously counted.
When the next print job is received, double-sided printing is executed by the processing from step S4. In this case, if the condition of stop time> predetermined time is satisfied, double-sided unit number = 2 is set in step S12, so double-sided printing is started in units of two sheets.

On the other hand, if the condition of stop time> predetermined time is not satisfied and the duplex unit sheet number is set to “1” in step S8 in the previous print job, the duplex unit sheet number = 1 remains. Double-sided printing is performed in units of one sheet.
As described above, when the number of printed pages exceeds the predetermined number P, the duplex mode is switched from the two-sheet unit mode to the one-sided duplex mode, so that the productivity can be improved as compared with the configuration in which the single-sided duplex mode is fixed. And the occurrence of tacking can be prevented.

  Further, if the stop time is within a predetermined time, the temperature of the sheet bundle stored in the first storage bin 51 is somewhat high, and when the sheets S by the next print job are stacked thereon, heat storage is performed. As a result, it is possible to prevent the occurrence of tacking by performing double-sided printing in the double-side mode in units of one sheet for the next print job. Further, if the stop time exceeds a predetermined time, the sheet bundle temperature is lowered and tacking does not occur, and the mode is returned to the double-sided mode in units of two sheets. Therefore, productivity is prevented while preventing the next print job from occurring. You can run without dropping.

In the above embodiment, an example in which the predetermined number P is set to 80 and the predetermined time T is set to 10 minutes has been described, but it goes without saying that the value such as the predetermined number P is not limited thereto. Considering the temperature of the discharged sheet S, the heat dissipation characteristics of the sheet bundle, the toner melting temperature, and the like, optimum values are determined in advance through experiments and the like, and the information is stored in the ROM 622 and the like.
In the above description, the sheet S is stored in the first storage bin 51. However, the same control can be performed when the second storage bin 52 is used instead of the first storage bin 51.

(Second Embodiment)
In the first embodiment, the predetermined condition for switching (returning) from the single-sheet unit to the double-sheet mode in units of two sheets is that the stop time exceeds the predetermined time T. In the present embodiment, The switching condition is that the sheet bundle stored in the first storage bin 51 is removed by the user, which is different from the first embodiment. Hereinafter, in order to avoid duplication of explanation, explanation of the same contents as those of the first embodiment is omitted, and the same constituent elements are denoted by the same reference numerals.

FIG. 6 is a diagram showing the contents of a flowchart in the control processing according to the present embodiment.
As shown in the figure, in this control process, the processes in steps S21 and S22 are performed between steps S2 and S3, and the other contents are the same as those in the first embodiment.
In step S21, it is determined whether or not the sheet bundle on the first storage bin 51 has been removed. The removal of the sheet bundle is determined by a detection signal from the sheet detection sensor 55.

If it is determined that the sheet bundle has been removed by the user (“YES” in step S21), the duplex unit number is reset to “2” and the number of printed pages is reset to “0” (step S22). ), The process proceeds to step S3.
In this case, when a print job is executed after step S3, duplex printing in the duplex mode of two sheets is started even if the condition of stop time> predetermined time is not satisfied. This is because if the sheet bundle is not accommodated in the first accommodation bin 51, it is not necessary to take into consideration the heat of the sheet bundle when it is accommodated, and the occurrence of tacking can be prevented.

On the other hand, if it is determined that the sheet bundle has not been removed, that is, it has been accommodated (“NO” in step S21), the process proceeds to step S3. In this case, the result is the same as that of the first embodiment, and only when the condition of stop time> predetermined time is satisfied, the mode is returned to the duplex mode in units of two sheets.
Thus, by setting the removal of the sheet bundle in a predetermined condition, even when the stop time is within a predetermined time, the mode can be switched to the double-side mode in units of two sheets, and productivity can be improved. The same control can be performed when the second storage bin 52 is used instead of the first storage bin 51.

(Third embodiment)
In the above embodiment, an example in which the accommodation destination of the discharged sheet S is the first accommodation bin 51 of the sorter 50 has been described. However, in the present embodiment, the first accommodation bin 51 and the second accommodation bin 52 are used. This point is different.
The accommodation destination is switched for each document. Specifically, for example, when there are documents A and B to be printed, the first storage bin 51 is selected when performing duplex printing on the document A, and the second storage when performing duplex printing on the document B. It is switched to the bin 51.

  FIG. 7 is a diagram showing the contents of a flowchart in the control processing according to the present embodiment. As shown in the figure, when power is first supplied to the printer 1 (step S1), the first storage bin duplex unit sheet number and the second storage bin duplex unit sheet number are set to “2”, and the first storage bin print surface is set. The number and the number of second storage bin print surfaces are reset to “0”, and the first storage bin non-discharge time and the second storage bin non-discharge time are reset to “0” (step S31).

Here, the first storage bin duplex unit number is the unit number M in the duplex mode when performing duplex printing when the first storage bin 51 is used, and the second storage bin duplex unit number. This is the unit number M in the duplex mode when performing duplex printing when the second storage bin 52 is used.
The number of first storage bin print surfaces is the total number of front and back surfaces of the sheet S by duplex printing when the first storage bin 51 is used. The second storage bin print surface number is the total number of the front and back surfaces of the sheet S by double-sided printing when the second storage bin 52 is used.

The first storage bin non-discharge time is an elapsed time after the double-sided printing is completed when the first storage bin 51 is used and duplex printing is executed. The second storage bin non-discharge time is an elapsed time after the double-sided printing is completed when the second storage bin 52 is used and the duplex printing is executed.
When a print job is received (“YES” in step S3), job registration is performed (step S4). Here, as the print job, for the above-described originals A and B, first, duplex printing of the original A is continuously performed on 160 sheets S, and each printed sheet S is stored in the first storage bin 51. When the double-sided printing of A is completed, an example of a job for performing the double-sided printing of the original B continuously on 160 sheets S and storing the printed sheets S in the second storage bin 52 will be described. .

In step S5, it is determined whether or not the current number of second bin bin print faces matches the value indicating the final face. Here, the 320th surface when the print instruction for the 160th sheet S for the document B is finished is the final surface.
If it is determined that the number of printed surfaces does not match the final surface (“NO” in step S5), it is determined which of the first storage bin 51 and the second storage bin 52 is selected as the storage destination (discharge destination). (Step S32). Here, the first storage bin 51 is selected until the duplex printing on the document A is completed.

In step S32, it is determined that the first storage bin 51 is selected, and the first storage bin print restriction process is executed (step S33).
FIG. 8 is a diagram showing a subroutine of the first storage bin print restriction process.
As shown in the figure, “1” is incremented to the current first storage bin print face number “0”, and the first storage bin non-discharge time is reset to “0” (step S311).

Then, the second storage bin non-discharge time is counted (step S312). Specifically, if the current second unaccommodated bin non-discharge time = 0, the count is started, and if it is currently being counted, the count is continued.
Next, it is determined whether or not the first number of bin print surfaces> the predetermined number of surfaces 1 (step S313). Here, the predetermined number of faces 1 corresponds to the predetermined number of faces in the first embodiment, and in this embodiment, from the two-sheet unit to the one-sheet unit during the duplex printing when the first storage bin 51 is used. Is used as a threshold value for determining whether or not to switch to the double-side mode, for example, 160 faces.

Here, since the current number of first storage bin print planes = 1, it is determined that the condition of first storage bin print plane number> predetermined number of planes 1 is not satisfied (“NO” in step S313), and step S315. Move on.
In step S315, it is determined whether or not second storage bin non-discharge time> predetermined time 2. Here, the predetermined time 2 corresponds to the predetermined time in the first embodiment, and in this embodiment, when the storage destination is switched from the first storage bin 51 to the second storage bin 52, two-sided mode is set. It is used as a threshold value for determining whether or not to return to the unit, for example, 20 minutes.

If it is determined that second storage bin non-discharge time> predetermined time 2 (“YES” in step S315), the second storage bin duplex unit sheet number = 2 and the second storage bin print surface count is reset to “0”. (Step S316) and the process returns to the main routine.
As a result, the next time the second storage bin 52 is used, double-sided printing is started in the double-side mode in units of two sheets. If the second storage bin non-discharge time> predetermined time 2, even if the sheet bundle remains in the second storage bin 52, the temperature of the sheet bundle is reduced to a level that does not cause tacking. This is because the occurrence of tacking can be prevented even if the productivity is increased in units of two sheets.

  On the other hand, if it is determined that second storage bin non-discharge time> predetermined time 2 is not satisfied, that is, second storage bin non-discharge time ≦ predetermined time 2 (“NO” in step S315), the process directly returns to the main routine. To do. It can be said that the temperature of the sheet bundle left in the second storage bin 52 has not yet decreased and the probability of occurrence of tacking is high. When the second storage bin 52 is used, the duplex mode for each sheet is set. It is for maintaining.

Returning to FIG. 7, in step S9, the engine control unit 61 is instructed to print the first sheet S on the first surface, and the process returns to step S5. Upon receiving the instruction, the engine control unit 61 feeds the first sheet S and executes printing on the first surface of the sheet S. Executing the instructed printing is the same as in the first embodiment.
If it is determined in step S5 that it is not the final surface and it is determined in step S32 that the storage destination is the first storage bin 51, the first storage bin print restriction process in step S33 is performed again.

In FIG. 8, in step S <b> 311, “1” is incremented to the current first storage bin print surface number “1”, and the first storage bin non-discharge time is reset to “0”.
In step S312, counting of the second storage bin non-discharge time is continued.
In step S 313, it is determined whether or not the number of first storage bin print planes> the predetermined number of planes 1. Since the current number of printed first bin bins = 2, it is determined that the condition that the number of first accommodated bin printed surfaces> predetermined number of 1 is not satisfied ("NO" in step S313), and the processing in steps S315 and S316 To return to the main routine.

In step S9 of FIG. 7, since the first storage bin duplex unit = 2 at present, the engine control unit 61 is instructed to print on the first surface of the second sheet S, and the process returns to step S5.
The processes in steps S5, S32, S33, and S9 are repeated until a print instruction for the 160th sheet S is issued for each side of the sheet S. Thus, double-sided printing is performed on the document A, and each printed sheet S is stored in the first storage bin 51.

In the meantime, up to the 80th sheet S is instructed to be printed based on the duplex mode in units of two sheets.
On the other hand, after the print instruction for the 80th sheet S is finished, if the first number of bins printed is set to 161 in step S311, the number of first bins printed is greater than the predetermined number 1 in step S313. It is determined that the condition is satisfied, and in step S314, the first storage bin duplex unit = 1 is set, and the process proceeds to step S315. As a result, for each of the 81st to 160th sheets S, a printing instruction based on the single-sided duplex mode is issued as in the first embodiment.

When the printing instruction for the 160th sheet S is completed (double-sided printing for the document A is completed), the accommodation destination is switched to the second accommodation bin 52, and the second accommodation bin print restriction process in step S34 is executed.
FIG. 9 is a diagram illustrating a subroutine of the second storage bin print restriction process.
As shown in the figure, in this process, the “first storage bin” in the first storage bin print restriction process is changed to “second storage bin”, the “second storage bin” is changed to “first storage bin”, “ “Predetermined number of faces 1” is replaced with “Predetermined number of faces 2”, and “Predetermined time 2” is replaced with “Predetermined time 1”. Here, the predetermined number of faces 2 is 200, for example, and the predetermined time 1 is, for example, 10 minutes. The reason why the predetermined number of faces 1 and 2 are different and the reason why the predetermined times 1 and 2 are different will be described later.

  For the first to 80th sheets S of the original B, duplex printing is performed in a duplex mode in units of 2 sheets (“NO” in steps S321, S322, and S323), and the 81st to 160th sheets. For S, double-sided printing is performed in the double-side mode in units of one sheet (“YES” in steps S321, S322, and S323, S324). Each sheet S after duplex printing is stored in the second storage bin 52.

Meanwhile, if it is determined that the first storage bin non-discharge time> predetermined time 1 (“YES” in step S325), the first storage bin duplex unit number of sheets = 2 is set and the first storage bin print surface number is “0”. (Step S326), and the process returns to the main routine.
As a result, when the first storage bin 51 is used next time, duplex printing is started in the duplex mode in units of two sheets. Even if the sheet bundle is left in the first storage bin 51, the temperature of the sheet bundle is lowered to a level that does not cause the occurrence of tacking, and the occurrence of tacking is prevented even if the productivity is increased by two sheets. Because it can.

  On the other hand, if it is determined that the first storage bin non-discharge time ≦ predetermined time 1 (“NO” in step S325), the process directly returns to the main routine. As a result, when the first storage bin 51 is used next time, double-sided printing is started in the single-sided duplex mode. It can be said that the temperature of the sheet bundle remaining in the first storage bin 51 has not yet decreased, and the probability of occurrence of tacking is high, and when the first storage bin 51 is used, the duplex mode for each sheet is maintained. It is to do.

Returning to FIG. 7, when the printing instruction for the 160th sheet S for the document B is finished, it is determined in step S5 that the current second storage bin print surface number matches the final surface, and step S35. Move on. This means the end of the print job.
In step S35, the first storage bin non-discharge time and the second storage bin non-discharge time are counted. If the current undischarge time is “0”, the counting is started, and if it is being counted, the counting is continued.

In step S36, it is determined whether or not the current first storage bin non-discharge time> predetermined time 1. If it is determined that the first storage bin non-discharge time> predetermined time 1 (“YES” in step S36), the first storage bin duplex unit sheet number = 2 is set and the first storage bin print surface number is set. Is reset to “0” (step S37), and the process proceeds to step S38.
On the other hand, if it is determined that the first storage bin non-discharge time> predetermined time 1 is not satisfied, that is, the first storage bin non-discharge time ≦ predetermined time 1 (“NO” in step S36), the process proceeds to step S38. . This is done for the same reason as when executing the processes of steps S325 and S326.

In step S38, it is determined whether or not the current second storage bin non-discharge time> predetermined time 2. If it is determined that second storage bin non-discharge time> predetermined time 2 (“YES” in step S38), the second storage bin duplex unit number of sheets = 2 is set, and the second storage bin print surface number is set to “ It resets to “0” (step S39) and returns to step S3.
On the other hand, if it is determined that second storage bin non-discharge time ≦ predetermined time 2 (“NO” in step S38), the process directly returns to step S3. This is done for the same reason as when the processes of steps S315 and S316 are executed.

  In step S3, when the next print job is received, the processing in step S4 and subsequent steps is performed, and the double-side printed sheet S is stored in the first storage bin 51 or the second storage bin 52. At this time, even if the sheet bundle of the previous print job is not removed by the user and remains in the first storage bin 51 and the second storage bin 52, the non-discharge time (elapsed time) is the predetermined time 1 or If the predetermined time 2 is exceeded, duplex printing is performed in the duplex mode in units of two sheets. Thereby, productivity is improved.

On the other hand, if the non-paper ejection time is within the predetermined time 1 or the predetermined time 2, double-sided printing is performed in the single-sided duplex mode, thereby preventing the occurrence of tacking.
Further, the transport paths 57 and 58 of the sorter 50 are different in distance (distance from the pair of reverse rollers 41 to the sheet discharge port). Specifically, the conveyance path 58 is longer than the conveyance path 57. Since the conveyance speed of the sheet S is the same regardless of which conveyance path is conveyed, the conveyance path 58 takes a longer time for conveyance, and is released from the sheet S being conveyed by the time difference. The amount of heat increases, and the temperature of the sheet S tends to decrease as the sheet is conveyed along the conveyance path 58.

  This means that the temperature of the sheet bundle stored in the second storage bin 52 is less likely to rise than that of the sheet bundle stored in the first storage bin 51, and when tacking occurs. It can be said that the value of the number P is larger in the second storage bin 52 than in the first storage bin 51. In other words, it can be said that the number of sheets P until the occurrence of tacking is greater in the second storage bin 52 than in the first storage bin 51.

Moreover, in this Embodiment, the thermal radiation characteristic of the accommodated sheet bundle differs for every accommodation bin. Specifically, the first storage bin 51 has better heat dissipation characteristics than the second storage bin 52, and therefore the time T requires that the second storage bin 52 requires a longer time than the first storage bin 51. It will be.
Therefore, in the present embodiment, the predetermined number of surfaces 2 and the predetermined time 2 for the second storage bin 52 are larger than the predetermined number of surfaces 1 and the predetermined time 1 for the first storage bin 51, specifically, the predetermined number of surfaces 1 = By setting the predetermined number of faces 2 = 200 for 160 faces and the predetermined time 2 = 20 minutes for the predetermined time 1 = 10 minutes, the occurrence of tacking is prevented and the productivity is further improved.

  Of course, it goes without saying that the predetermined number of faces and the predetermined time value for each storage bin are not limited to those described above. An optimum value is determined in advance by experiments or the like according to the distance of the transport path of the sorter 50, heat radiation characteristics, and the like. In that case, an example in which the vertical relationship between the predetermined number of faces and the value of the predetermined time for each storage bin is reversed may be included. In addition, although the example of a structure of the sorter provided with two accommodation bins was demonstrated above, if it is the structure with two or more sheet accommodation bins (sheet accommodation trays), it will not be restricted to two but three or more may be sufficient. An optimal predetermined number of surfaces or the like is determined for each storage bin.

  Note that the present invention is not limited to the image forming apparatus, and may be an image forming method for switching the duplex unit number. 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.

  Further, the program according to the present invention does not have to include all modules for causing the computer to execute the processing described above. For example, a separate information process such as a communication program or a program included in an operating system (OS) is performed. You may make it make a computer perform each process of this invention using the various general purpose programs which can be installed in an apparatus. Accordingly, it is not always necessary to record all the modules on the recording medium of the present invention, and it is not always necessary to transmit all the modules. Further, there are cases where predetermined processing can be executed using dedicated hardware.

(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-described embodiment, the example of the printer 1 that maximizes the duplex unit sheet number M = 2 has been described as the configuration in which the duplex printing is performed while circulating a plurality of sheets. Needless to say, it is not limited to two. For example, the present invention can be applied to a printer that maximizes M = 3. In this case, double-sided printing is performed in the duplex mode in which the unit number of sheets M is 3 sheets from the first sheet up to a predetermined number of pages (hereinafter described in place of “number of sheets”). A configuration is possible in which the printing is switched to a duplex value in a small value, for example, the duplex mode of two sheets or one sheet.

  Further, it is possible to adopt a configuration in which the duplex mode is switched in stages with a plurality of thresholds when the duplex mode is switched. For example, when M = 3, double-sided printing is performed in the duplex mode of M = 3 from the first sheet to the first predetermined number, and after the first predetermined number is exceeded, the second predetermined number (> first Can be switched to the duplex mode of M = 2 until the second predetermined number of sheets, and further switched to the duplex mode of M = 1.

  Further, M = 1 or 2 when stop time (undischarged time) ≦ predetermined time, and M = 3 when stop time (undischarged time)> predetermined time. Further, when the first predetermined time T1 <the second predetermined time T2, when M1 is the stop time (non-paper discharge time) ≦ T1, T1 <the stop time (non-paper discharge time) ≦ T2. In this case, M = 2, and in the case of T2 <stop time (undischarged time), M = 3.

(2) The predetermined number of sheets is not limited to be constant, and may be changed according to image forming conditions. The image forming condition can be, for example, the type of sheet. Specifically, sheets of a plurality of thickness as a sheet to be used, specifically plain paper (60~90g ^{/ m} 2), the cardboard 1 (90~150g ^{/ m} 2), thick paper ² (150g / ^m 2 In such a configuration that the sheet can be selectively fed, the predetermined number 1 <predetermined number 2 <predetermined number 3 can be set in this order. As the sheet thickness (weight) increases, the heat capacity stored in one sheet increases, and the heat of the sheet bundle becomes difficult to dissipate. As the sheet thickness to be used becomes thicker, it is desirable to reduce the threshold value (the value of the predetermined number) when switching the duplex mode.

  Also, for example, the sheet size can be used. This is because as the sheet size increases, the heat capacity stored in one sheet increases and the heat of the sheet bundle becomes difficult to dissipate. It is conceivable to reduce the value of the predetermined number as the sheet size increases. Note that the sheet thickness and sheet size can be specified by a method set by the user by key input or the like, a detection means such as a sensor for detecting the thickness or size, and a determination method based on the detection result. .

  Further, for example, the B / W ratio (corresponding to the printing rate, the area of the printed portion with respect to the area of one sheet) at the time of double-sided printing can also be used. A large B / W ratio means that the amount of heat applied at the time of fixing does not change, but the area of the image printed on each sheet S is large, so two sheets of sheets stacked in the storage bin overlap. This is because the ratio of contact between the toner image printed on the back surface of the upper sheet S and the toner image printed on the front surface of the lower sheet S is increased, and the probability of occurrence of tacking is considered to increase. . In this case, it is preferable to decrease the value of the predetermined number as the B / W ratio increases. The B / W ratio is specified from a method of calculating the ratio of the total number of pixels of one sheet to the number of pixels corresponding to the image portion in image processing, a method set by a user by key input, or the like. Can do.

  (3) In the above-described embodiment, duplex printing is performed in the duplex mode in which M = 1 from the predetermined number of sheets to the last sheet. However, it is possible to return to the duplex mode of M = 2 in the middle, for example. If the temperature of the sheet bundle after discharge can be lowered to some extent by the duplex mode of M = 1, the productivity can be improved by returning to the duplex mode of M = 2. Of course, after returning to M = 2, if the temperature of the sheet bundle rises until it approaches the temperature at which tacking occurs, it is switched to M = 1 again before that. The threshold value for returning to M = 2 and switching to M = 1 again can be determined in advance through experiments or the like. Further, a detection unit such as a sensor for detecting the temperature of the sheet bundle may be provided, and the duplex mode may be switched based on the detection result of the detection unit.

  (4) In the above embodiment, an example in which the image forming apparatus according to the present invention is applied to a printer has been described. However, heat fixing is performed by forming an image such as a toner image on the first and second surfaces of a sheet. The image forming apparatus can be applied to a copying machine, a facsimile machine, an MFP (Multiple Function Peripheral), and the like. Moreover, although the example which used the sorter as a post-processing part was demonstrated, if it is provided with the accommodation bin (tray) which accommodates the discharged | emitted sheet | seat, it will not be restricted to this, For example, a finisher etc. can also be used.

  Further, the above embodiment and the above modification examples may be combined.

  The image forming apparatus according to the present invention is an apparatus that forms an image on both sides of a sheet and is useful as a technique for preventing the occurrence of tacking.

1 is a diagram illustrating an overall configuration of a printer 1 according to a first embodiment. 2 is a diagram illustrating a configuration of a control unit 60 of the printer 1. FIG. It is a figure which shows the example of the experimental result which shows the presence or absence of the occurrence of tacking when printing by double-sided mode is performed on the conditions of pattern AD using the printer. FIG. 10 is a diagram illustrating a content of a flowchart in a control process when a controller unit 62 of the control unit 60 executes a print job in a duplex mode. It is a figure which shows a timing chart when printing is instruct | indicated from the controller part 62 to the engine control part 61. FIG. It is a figure which shows the content of the flowchart in the control processing which concerns on 2nd Embodiment. It is a figure which shows the content of the flowchart in the control processing which concerns on 3rd Embodiment. It is a figure which shows the subroutine of the 1st accommodation bin print restriction | limiting process in the said control processing. It is a figure which shows the subroutine of the 2nd accommodation bin print restriction | limiting process in the said control processing.

Explanation of symbols

DESCRIPTION OF SYMBOLS 1 Printer 10 Image process part 15 Secondary transfer position (image formation position)
27 Conveying path 30 Fixing section 40 Double-sided conveying section 45 Circulating conveying path 50 Sorter (post-processing section)
51 First Storage Bin 52 Second Storage Bin 55, 56 Seat Detection Sensor 60 Control Unit 61 Engine Control Unit 62 Controller Unit S Seat

Claims (4)

M sheets (M is an integer of 2 or more) sheets of conveyed to the image forming position, one by one, while sequentially performs image formation by the toner on the first surface of the sheet, the toner image formed on the first surface After the image is heat-fixed, the sheet is reversed and conveyed one by one through the circulation conveyance path, and after completion of image formation on the first surface of all M sheets, each sheet conveyed through the circulation conveyance path is An image forming operation is performed in units of M sheets, which is transported again to the image forming position, sequentially forms an image with toner on the second surface of the sheet, and discharges the sheet after the toner image formed on the second surface is thermally fixed. An image forming unit that can be executed by switching between the first double-sided mode that is repeatedly performed in step 2 and the second double-sided mode in which an image forming operation is performed by changing the value of M to a value of 1 or more and (M−1) or less;
When forming an image on N sheets (N is an integer greater than M), from the first sheet to the Pth sheet (P is a predetermined value, an integer greater than M and smaller than N) Is set to the first double-sided mode, and the image forming unit executes the image forming operation in the first double-sided mode until the Pth sheet, and the (P + 1) th and subsequent sheets are Control means for executing the image forming operation by switching to the second duplex mode ;
An accommodating portion for accommodating the discharged sheet;
Detecting means for detecting that the sheet accommodated in the accommodating portion has been removed ,
The control means includes
After the switching to the second duplex mode, when the detection unit detects that the sheet stored in the storage section has been removed, the subsequent image forming operation is performed in the first duplex mode. image forming apparatus characterized by causing. M sheets (M is an integer of 2 or more) sheets of conveyed to the image forming position, one by one, while sequentially performs image formation by the toner on the first surface of the sheet, the toner image formed on the first surface After the image is heat-fixed, the sheet is reversed and conveyed one by one through the circulation conveyance path, and after completion of image formation on the first surface of all M sheets, each sheet conveyed through the circulation conveyance path is An image forming operation is performed in units of M sheets, which is transported again to the image forming position, sequentially forms an image with toner on the second surface of the sheet, and discharges the sheet after the toner image formed on the second surface is thermally fixed. An image forming unit that can be executed by switching between the first double-sided mode that is repeatedly performed in step 2 and the second double-sided mode in which an image forming operation is performed by changing the value of M to a value of 1 or more and (M−1) or less;
When forming an image on N sheets (N is an integer greater than M), from the first sheet to the Pth sheet (P is a predetermined value, an integer greater than M and smaller than N) Is set to the first double-sided mode, and the image forming unit executes the image forming operation in the first double-sided mode until the Pth sheet, and the (P + 1) th and subsequent sheets are Control means for executing the image forming operation by switching to the second duplex mode ;
A plurality of storage bins including a first storage bin and a second storage bin; a post-processing unit capable of storing the discharged sheet by switching to any of the plurality of storage bins ;
The control means includes
When the storage destination of the discharged sheet is switched from the first storage bin to the second storage bin after switching to the second duplex mode, the subsequent image forming operation is performed on the first duplex surface. An image forming apparatus , wherein the sheet is executed in the mode and the sheet after image formation in the first double-side mode is stored in the second storage bin . M sheets (M is an integer of 2 or more) sheets of conveyed to the image forming position, one by one, while sequentially performs image formation by the toner on the first surface of the sheet, the toner image formed on the first surface After the image is heat-fixed, the sheet is reversed and conveyed one by one through the circulation conveyance path, and after completion of image formation on the first surface of all M sheets, each sheet conveyed through the circulation conveyance path is An image forming operation is performed in units of M sheets, which is transported again to the image forming position, sequentially forms an image with toner on the second surface of the sheet, and discharges the sheet after the toner image formed on the second surface is thermally fixed. An image forming unit that can be executed by switching between the first double-sided mode that is repeatedly performed in step 2 and the second double-sided mode in which an image forming operation is performed by changing the value of M to a value of 1 or more and (M−1) or less;
When forming an image on N sheets (N is an integer greater than M), from the first sheet to the Pth sheet (P is a predetermined value, an integer greater than M and smaller than N) Is set to the first double-sided mode, and the image forming unit executes the image forming operation in the first double-sided mode until the Pth sheet, and the (P + 1) th and subsequent sheets are Control means for executing the image forming operation by switching to the second duplex mode ;
A plurality of storage bins including a first storage bin and a second storage bin; a post-processing unit capable of storing the discharged sheet by switching to any of the plurality of storage bins ;
The control means includes
2. The image forming apparatus according to claim 1, wherein the value of P is different between when the sheet is stored in the first storage bin and when the sheet is stored in the second storage bin . The control means includes
A controller unit for instructing an image forming operation in the duplex mode;
An engine control unit that receives an instruction from the controller unit and controls an image forming operation of the image forming unit;
The image forming apparatus according to any one of claims 1 to 3, characterized in that it comprises a.

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