JP5523617B2 - Image forming apparatus - Google Patents

Description

  The present invention relates to an image forming apparatus, and more particularly, to drive control of a fan for cooling a sheet.

  2. Description of the Related Art Conventionally, in an image forming apparatus such as a copying machine or a printer that forms an image using an electrophotographic method, a toner image is transferred to a sheet, and then the sheet is conveyed to a fixing device to fix the toner image. An image is formed on the top. Further, in such an image forming apparatus, after the sheet on which the image is formed is reversed by the reversing unit, the sheet is conveyed again to the image forming unit by the reconveying unit, and image formation is performed on both the front and back sides of the sheet. Some have an image forming mode.

  By the way, in such a conventional image forming apparatus, a sheet is discharged onto a paper discharge tray after fixing. In this case, the sheet may not be sufficiently cooled. In this case, a phenomenon in which sheets discharged onto the paper discharge tray stick to each other with melted toner (hereinafter referred to as a blocking phenomenon) may occur. As a countermeasure, for example, there is a cooling unit that contacts the cooling air along the sheet stacking direction so as to lower the temperature of the sheet discharged onto the sheet discharge tray (see Patent Document 1). . In addition, there is a fan in which a fan is disposed above the sheet discharge tray, the necessity of air cooling for the stacked sheets is determined, and the fan is operated / stopped as necessary (see Patent Document 2). With this configuration, the fan can be stopped under conditions that do not cause a blocking phenomenon on the output tray, such as when the number of stacked sheets is small or the sheet spacing is wide, and noise and power can be stopped. Consumption can be minimized.

  FIG. 6 is a view for explaining the air flow by the fan of the conventional image forming apparatus in which the sheets stacked on the paper discharge tray are cooled by the fan. In this image forming apparatus, a sheet on which an image is formed on one side (first side) after passing through the fixing device 300 is normally discharged to a paper discharge tray 310. When the sheets are discharged to the discharge tray 310 in this way, the cooling air indicated by the arrow between the discharged sheet S1 and the already discharged sheet Sa by the fan 402 as shown in FIG. I try to flow. In addition, by causing the cooling air to flow between the discharged sheet S1 and the already discharged sheet Sa in this way, it is possible to prevent the occurrence of a blocking phenomenon that is a sticking of the sheet.

  In the double-sided image forming mode in which images are formed on both the front and back sides of the sheet, the sheet on which the image is formed on the first side and passed through the fixing device 300 is guided to the drawing path 301. Then, after the sheet S guided to the pull-in path 301 is guided to the reversing path 305 by the normal rotation of the reversing rollers 302 and 303, the sheet S is fed by the reverse rotation of the reversing roller 303 with the trailing edge at the leading edge as the head. Send in the opposite direction. In addition, the sheet sent out in the state where the front-rear direction is reversed in this way is sent to the double-sided conveyance path 306 while being guided by a guide member (not shown), and is then conveyed again to the image forming unit and is transferred to the back surface. The toner image is transferred. Then, after passing through the fixing device 300, the sheet S1 on which the image is formed on the back surface is discharged to the discharge tray 310. Then, by causing the cooling air to flow between the sheet S1 discharged to the discharge tray 310 and the already discharged sheet Sa in this way, the occurrence of the blocking phenomenon can be prevented.

  By the way, this image forming apparatus is provided with a reverse discharge mode for discharging the sheet that has passed through the fixing device 300 to the discharge tray 310 by reversing the front and back and the front-rear direction in order to align the page order of the discharged sheets. Yes. In such a reverse discharge mode, after the sheet is guided to the pull-in path 301, the sheet is conveyed to the reverse discharge path 307 by the reverse rotation of the reverse rollers 302 and 303 and the switching of the guide member. Discharge.

JP 2007-219399 A JP 2007-079310 A

  However, in such a conventional image forming apparatus, the lead-in path 301 branches downward as shown in FIG. For this reason, in the case of reverse paper discharge, the sheet immediately after passing through the fixing device 300 passes through the curved pull-in path 301, and at this time, curling due to the downward curve of the pull-in path 301 occurs. . When the curled sheet is reversed and discharged, the front and back sides of the sheet are reversed, and the front end and the rear end in the discharge direction of the discharge tray 310 are curled upward. Curled. Note that when the sheet passes through the reverse discharge path 307 and is discharged to the discharge tray 310, the sheet is cooled while being affected by the curvature of the pull-in path 301 immediately after passing through the fixing device 300. Thus, the curved state of the reverse paper discharge path 307 is hardly affected. In this state, when the cooling air is caused to flow between the discharged sheet and the discharged sheet by the fan 402, the cooling air indicated by the arrow is discharged onto the discharge tray 310 as shown in FIG. It enters the lower surface of the upper curled sheet S1. As a result, the sheet S1 floats and collides with the succeeding sheet S2, and as a result, the stackability on the paper discharge tray 310 is impaired.

  Further, in a configuration in which the drawing path branches upward, curling due to upward bending of the drawing path occurs. When the curled sheet is reversed and discharged, the front and back sides of the sheet are reversed, and the lower end curl state in which the front end portion and the rear end portion of the discharge tray 310 are curved downward. It becomes. In this state, when cooling air is caused to flow between the discharged sheet and the already discharged sheet by the fan 402, the cooling air is received at the front end in the sheet discharge direction of the discharged sheet S1 in the lower curled state. It is carried downstream from the paper discharge position in the paper discharge direction. As a result, the leading end of the sheet S1 in the sheet discharge direction hangs down from the sheet discharge tray 310, and there is a possibility that the sheet S1 cannot return to an appropriate stacking position due to the inclination of the stacking surface of the sheet discharge tray 310. If a decurling unit for correcting the upper curl and lower curl of the sheet is provided in order to maintain the stackability of the sheet, the apparatus is increased in size and cost.

  SUMMARY An advantage of some aspects of the invention is that it provides an image forming apparatus capable of preventing sheet sticking and maintaining sheet stackability. is there.

  The present invention provides a fixing unit that fixes a toner image on a sheet, a discharge unit that discharges the sheet on which the toner image is fixed to a sheet stacking unit, and a sheet stacking unit that fixes the sheet on which the toner image is fixed in the image forming apparatus. A discharge passage that guides the air to the sheet, and a blowout port that is provided between the discharge unit and the sheet stacking unit and blows air between the sheet discharged by the discharge unit and the sheet that has already been discharged to the sheet stacking unit. And a blower that blows air blown from the blower outlet toward the blower outlet, and a control unit that controls the blowing operation of the blower unit, and the control unit is a sheet that has already been imaged on one side. When a sheet having a toner image fixed on the back side opposite to the one side is discharged toward the sheet stacking unit, the toner amount of the toner image fixed on the back side exceeds a predetermined amount. When on actuating the said blowing means, when the amount of toner in the fixed toner image on the back surface is less than a predetermined amount is characterized in that not to operate the blower means.

  As in the present invention, the air blowing means is operated when the toner amount of the toner image formed on the back surface exceeds a predetermined amount, and the air blowing means is not operated when the toner amount is less than the predetermined amount. It is possible to prevent sticking and maintain sheet stackability.

1 is a diagram showing a schematic configuration of a color laser printer which is an example of an image forming apparatus according to a first embodiment of the present invention. FIG. 3 is a control block diagram of the color laser printer. 5 is a flowchart showing operation control of a fan of the color laser printer. FIG. 6 is a control block diagram of an image forming apparatus according to a second embodiment of the present invention. 6 is a flowchart showing operation control of a fan of the image forming apparatus. The figure explaining the flow of the air by the fan of the conventional image forming apparatus.

  Hereinafter, embodiments for carrying out the present invention will be described in detail with reference to the drawings. FIG. 1 is a diagram showing a schematic configuration of a color laser printer which is an example of an image forming apparatus according to a first embodiment of the present invention. In FIG. 1, reference numeral 1 denotes a color laser printer, and 1A denotes a color laser printer main body (hereinafter referred to as a printer main body). The printer main body 1A is provided with an image forming unit 1B that forms an image on the sheet S, an intermediate transfer unit 1C, a fixing device 5, and a sheet feeding device 1D that feeds the sheet S to the image forming unit 1B. Yes. The color laser printer 1 can form an image on the back surface of the sheet. For this reason, the sheet S on which the image is formed on the front surface (first surface) is reversed and image formation is performed again. A re-conveying unit 1E for conveying to the unit 1B is provided.

  The image forming unit 1B is arranged in a substantially horizontal direction, and each of the four process stations 2 (2Y, 2Y, 2M, 4M) forms four color toner images of yellow (Y), magenta (M), cyan (C), and black (Bk). 2M, 2C, 2K). This process station 2 carries toner images of four colors, yellow, magenta, cyan and black, and is a photosensitive drum 11 (11Y, 11M, 12C, 11K) which is an image carrier driven by a stepping motor (not shown). ). Further, a charging device 12 (12Y, 12M, 12C, 12K) for uniformly charging the surface of the photosensitive drum is provided. Furthermore, an exposure device 13 (13Y, 13M, 13C, 13K) is provided that forms an electrostatic latent image on a photosensitive drum that rotates at a constant speed by irradiating a laser beam based on image information. In addition, a developing device 14 (14Y, 14M, 14C, 14K) is provided that visualizes a toner image by attaching yellow, magenta, cyan, and black toner to the electrostatic latent image formed on the photosensitive drum. Yes. The charging device 12, the exposure device 13, the developing device 14 and the like are arranged around the photosensitive drum 11 along the rotation direction.

  The sheet feeding device 1D is provided at the lower part of the printer main body, and is a sheet feeding cassette 61-64 that is a sheet storage unit that stores the sheet S, and a pickup roller 71 that feeds the sheet S stacked and stored in the sheet feeding cassette 61-64. -74. When the image forming operation is started, the sheets S are separated and fed one by one from the sheet feeding cassettes 61 to 64 by the pickup rollers 71 to 74, and then pass through the conveyance vertical path 81 to the registration roller 76. Be transported. Here, the registration roller 76 has a function of correcting skew by following the leading edge of the sheet S by creating a loop by the sheet S being abutted. Further, it has a function of conveying the sheet S to the secondary transfer unit at a predetermined timing in accordance with the timing of image formation on the sheet S, that is, a toner image carried on an intermediate transfer belt described later. . When the sheet S is conveyed, the registration roller 76 is stopped, and the sheet S is abutted against the registration roller 76 in such a stopped state, so that the sheet is bent. Thereafter, the skew of the sheet S is corrected by the leading edge of the sheet being aligned with the nip of the registration roller 76 due to the rigidity of the sheet S. Thereafter, when the skew of the sheet S is corrected, the registration roller 76 is driven at a timing at which the toner image formed on the intermediate transfer belt 31 coincides with the leading edge of the sheet S as described later.

  The intermediate transfer unit 1 </ b> C includes an intermediate transfer belt 31 that is driven to rotate along the arrangement direction of the process stations 2 indicated by arrows in synchronization with the outer peripheral speed of the photosensitive drum 11. Here, the intermediate transfer belt 31 applies an appropriate tension to the intermediate transfer belt 31 by the urging force of the driving roller 33, the driven roller 32 that forms the secondary transfer region with the intermediate transfer belt 31 interposed therebetween, and a spring (not shown). The tension roller 34 is stretched. The intermediate transfer belt 31 includes four primary transfer rollers 35 (35Y, 35M, 35C, and 35K) that constitute the primary transfer unit by sandwiching the intermediate transfer belt 31 together with the photosensitive drum 11 on the inside. ing. The primary transfer rollers 35 are connected to a transfer bias power source (not shown). Then, by applying a transfer bias from the primary transfer roller 35 to the intermediate transfer belt 31, each color toner image on the photosensitive drum is sequentially transferred to the intermediate transfer belt 31 and a full color image is formed on the intermediate transfer belt 31. Is done.

  A secondary transfer roller 41 is disposed so as to face the driven roller 32, and the secondary transfer roller 41 contacts the lowermost surface of the intermediate transfer belt 31 and is conveyed by the registration roller 76. S is nipped and conveyed together with the intermediate transfer belt 31. When the sheet S passes through the nip portion between the secondary transfer roller 41 and the intermediate transfer belt 31, a bias is applied to the secondary transfer roller 41, so that the toner image on the intermediate transfer belt is transferred to the sheet S on the secondary side. Transcribed. The fixing device 5 constituting the fixing unit fixes the toner image formed on the sheet on the sheet S via the intermediate transfer belt 31, and the sheet S holding the toner image passes through the fixing device 5. At this time, the toner image is fixed by applying heat and pressure.

  Next, an image forming operation of the color laser printer 1 configured as described above will be described. When the image forming operation is started, first, in the process station 2Y which is the most upstream in the rotation direction of the intermediate transfer belt 31, the exposure device 13Y irradiates the photosensitive drum 11Y with laser, and the yellow on the photosensitive drum. The latent image is formed. Thereafter, the developing device 14Y develops the latent image with yellow toner to form a yellow toner image. Next, the yellow toner image thus formed on the photosensitive drum 11Y is primarily transferred to the intermediate transfer belt 31 in the primary transfer region by the primary transfer roller 35Y to which a high voltage is applied.

  Next, the toner image is transferred together with the intermediate transfer belt 31 by the photosensitive drum 11M and the primary transfer roller 35M of the next process station 2M where the image is formed with a delay by the time the toner image is conveyed from the process station 2Y. It is conveyed to the configured primary transfer area. Then, the next magenta toner image is transferred onto the yellow toner image on the intermediate transfer belt by aligning the leading end of the image. Thereafter, the same process is repeated, and as a result, the four color toner images are primarily transferred on the intermediate transfer belt 31 to form a full-color image on the intermediate transfer belt. Note that the transfer residual toner slightly remaining on the photosensitive drum is collected by the photosensitive cleaner 15 (15Y, 15M, 15C, 15K), and prepared for the next image formation again.

  In parallel with the toner image forming operation, for example, the sheets S accommodated in the sheet feeding cassettes 61 to 64 are separated and fed one by one by the pickup rollers 71 to 74 and then conveyed to the registration roller 76. . At this time, the registration roller 76 is stopped, and the skew of the sheet S is corrected by abutting the sheet S against the registration roller 76 in the stopped state. Further, after the skew is corrected, the sheet S is transferred to the secondary transfer roller 41 and the intermediate transfer roller 41 by the registration roller 76 which starts rotating at the timing when the leading edge of the sheet coincides with the toner image formed on the intermediate transfer belt 31. It is conveyed to the nip portion with the belt 31. Then, the sheet is nipped and conveyed by the secondary transfer roller 41 and the intermediate transfer belt 31, and the sheet is applied by the bias applied to the secondary transfer roller 41 when passing through the nip portion between the secondary transfer roller 41 and the intermediate transfer belt 31. The toner image on the intermediate transfer belt is secondarily transferred to S.

  Next, the sheet S on which the toner image has been secondarily transferred is conveyed to the fixing device 5 by the pre-fixing conveying device 42. Then, the fixing device 5 melts and fixes the toner image on the sheet S by applying a predetermined pressing force by an opposing roller or belt or the like and generally a heating effect by a heat source such as a heater. Here, the color laser printer 1 includes a face-up mode in which a sheet on which an image is formed is discharged onto a discharge tray with the image forming surface facing up, a face-down mode in which the sheet is discharged with the image forming surface facing down, It has an automatic duplex mode that forms images on both the front and back sides of the sheet. In the face-up mode, the sheet S having the fixed image is not transported to the discharge transport path 82, and in the automatic duplex mode and the face-down mode, the sheet S having the fixed image is not transported to the pull-in path 83. The route is selected by the illustrated switching member.

  Here, in the face-up mode, which is one of the discharge modes, the sheet S having a fixed image passes through a discharge conveyance path 82 that is a discharge passage, and is discharged by a discharge roller 77 that is a discharge member. Are discharged to the paper discharge tray 65. In the case of the automatic duplex mode, which is another discharge mode, the sheet S passes through a pull-in path 83 branched downward from the discharge conveyance path 82 and is switched by a first reverse roller pair 78 and a second reverse roller pair 79 to a switchback path 84. Drawn into. Thereafter, by performing a switchback operation for reversing the second reversing roller pair 79 from the normal rotation, the leading and trailing ends are switched, and the sheet is conveyed to the duplex conveying path 85. Thereafter, the sheet S joins the conveyance vertical path 81 in synchronization with the sheet S of the subsequent job conveyed by the pickup rollers 71 to 74, and is similarly sent to the secondary transfer unit via the registration roller 76. It is done. The subsequent image forming process for the back surface (second surface) is the same as that for the front surface (first surface) described above. In the face-down mode, which is a reverse discharge mode for reversely discharging the sheet S, the sheet S that has passed through the fixing device 5 is discharged to the discharge tray 65 with the front and back sides and the front-rear direction reversed. Yes. Therefore, when the face-down mode is selected, the sheet S that has passed through the fixing device 5 is switched back through the curved retraction path 83 by the normal rotation of the first reverse roller pair 78 and the second reverse roller pair 79. It is drawn to 84. Then, the drawn sheet S is conveyed in the direction opposite to the feeding direction starting from the rear end when fed by the reverse rotation of the first reversing roller pair 78 and the second reversing roller pair 79 to be reversed and discharged. The paper is sent to the paper path 90 and discharged to the paper discharge tray 65 by the paper discharge roller 77. In the present embodiment, the drawing path for guiding the sheet to the paper discharge tray 65 by reversing the front and back and the leading and trailing ends of the sheet in the sheet conveyance direction after the sheet on which the toner image is fixed is pulled in is a drawing path 83 and a switch. The back path 84 and the reverse paper discharge path 90 are configured.

  In FIG. 1, reference numeral 88 denotes a paper discharge sensor provided on the upstream side of the paper discharge roller 77 in the sheet conveying direction. The paper discharge sensor 88 is provided at a position where the sheet before being discharged onto the paper discharge tray 65 can be detected in any of the face-up mode, face-down mode, and automatic double-side mode. . In the present embodiment, when the sheet S passes, the sheet discharge sensor 88 detects the leading edge of the sheet S by moving a detection piece (not shown) to shield a photo interrupter (not shown). is there. In FIG. 1, reference numeral 86 denotes a fan that is provided below the paper discharge tray 65 and serves as a blowing unit that sucks air outside the printer main body. Then, the air sucked by the fan 86 is blown to a duct 87 installed in the printer main body, and is directed toward the sheet S on the paper discharge tray 65 from the outlet 1F formed below the paper discharge roller 77. Blown out. Thus, the air blown out from the blower outlet 1F is blown out between the discharged sheet and the discharged sheet Sa, thereby preventing the occurrence of a blocking phenomenon.

  FIG. 2 is a control block diagram of the color laser printer 1, and a CPU (control unit) 89 provided at a predetermined position of the printer main body 1A includes, for example, an operation unit 100 disposed on the upper surface of the printer main body 1A, and a discharge unit. A paper sensor 88 and a fan 86 are connected. In addition, a face-up mode signal, a face-down mode signal, and an automatic duplex mode signal are input to the CPU 89 from the operation unit 100 or an external PC (not shown). The CPU 89 controls the air blowing operation by selectively driving the fan 86 according to the input mode. Here, in the face-down mode, since the sheet passes through the switchback path 84 and the reverse discharge path 90, the conveyance distance from the fixing device 5 to the discharge tray 65 is larger than that in the face-up mode and the automatic duplex mode. As a result, the natural cooling effect is increased. For this reason, the blocking phenomenon does not occur without cooling by the fan 86. For this reason, in the present embodiment, when the input mode is the face-down mode, the CPU 89 does not operate the fan 86 and puts it into a stopped state. As described above, in the face-down mode, by not operating the fan 86, the sheet floats even if the sheet is curled upward due to the influence of the pull-in path 83 curved immediately below the fixing device 5. Therefore, the stackability of the sheets is not impaired.

  Next, fan operation control in the present embodiment will be described with reference to the flowchart shown in FIG. First, a case where an image is formed on one side of the sheet S and the sheet is discharged face up will be described. In this case, when the image forming job is started, the CPU 89 first recognizes whether the mode designated in advance by the operation unit 100 or an external PC (not shown) is the face-down mode (S1). Here, when the face-up mode is designated, that is, when the designated mode is not the face-down mode (N in S1), the sheet S is subsequently conveyed, and the paper discharge sensor 88 detects the leading edge of the sheet S. (S5), the fan 86 is operated (S6). Thereby, air is blown out between the discharged sheet and the already discharged sheet Sa from the outlet 1F, thereby preventing the occurrence of a blocking phenomenon. Next, it is determined whether the job is completed (S4). Until the job is completed (N in S4), the fan 86 continues to operate in the face-up mode for the second and subsequent sheets. When the job is finished (Y in S4), the fan 86 is stopped (S7).

  Next, the case of the automatic duplex mode in which images are formed on both sides of the sheet S will be described. In the automatic duplex mode, that is, when the designated mode is not the face-down mode (N in S1), as described above, an image is formed on the second surface of the sheet S and the discharge sensor 88 detects the leading edge of the sheet S. (S5), the fan 86 is operated (S6). Even in the second and subsequent sheets, the fan 86 continues to operate in the automatic duplex mode, and when the job is finished (Y in S4), the fan is stopped (S7). On the other hand, when the designated mode is the face-down mode (Y in S1), even if the leading edge of the sheet S being conveyed is detected by the paper discharge sensor 88 (S2), the fan 86 is not activated and is set in the stopped state (S3 ). Next, it is determined whether or not the job is completed (S4). Until the job is completed (N in S4), if the second and subsequent sheets are also in face-down mode, the fan 86 is stopped (S3).

  By the way, in this embodiment, the face-up mode, the face-down mode, and the automatic duplex mode may be mixed in one job. For example, the first sheet may be in face-up mode, the second sheet may be in face-down mode, and the third sheet may be in automatic duplex mode. In this case, that is, when the first sheet is not in the face-down mode (N in S1), when the paper discharge sensor 88 detects the leading edge of the first sheet S (S5), the fan 86 is activated (S6). Since the second sheet is in the face down mode (Y in S1), in this case, when the paper discharge sensor 88 detects the leading edge of the second sheet S (S2), the fan 86 is stopped (S3). Since the third sheet is in the automatic double-side mode, in this case, that is, when the third sheet is not in the face-down mode (N in S1), when the paper discharge sensor 88 detects the leading edge of the third sheet S (S5), The fan 86 is operated (S6). For the fourth and subsequent sheets, the fan 86 is operated and stopped depending on the mode, and the fan 86 is stopped at the end of the job (S7).

  As described above, in the present embodiment, the fan 86 is operated in the face-up mode and the automatic duplex mode that are the discharge mode, and the fan 86 is stopped in the face-down mode that is the reverse discharge mode. Yes. By controlling in this way, it is possible to maintain sheet stackability while preventing the blocking phenomenon in all modes. Note that the present invention is also effective in a configuration in which the drawing path 83 immediately after the fixing device 5 is curved upward. Even if the sheet is curled downward due to the upward curving path immediately after the fixing device 5, the fan is stopped, so that the sheet does not fly too much, and the sheet stackability is not impaired.

  Next, a second embodiment of the present invention will be described. FIG. 4 is a control block diagram of the image forming apparatus according to the present embodiment. In FIG. 4, the same reference numerals as those in FIG. 2 described above indicate the same or corresponding parts. In the present embodiment, a manual double-side mode is provided which is a back-side mode in which an image is formed on the back side (second side) of a sheet on which an image is already formed on one side (first side). When performing this manual duplex mode, the user sets a sheet in the sheet storage of the image forming apparatus with the surface on which an image has already been formed facing upward. Subsequently, the manual duplex mode is set by the operation unit 100. Here, in the manual duplex mode, an image is already formed on one side. For this reason, when a sheet is discharged in the face-down mode in which the page order is aligned, if the amount of toner in the newly formed image is large, the image is newly formed on the back side and formed on one side of the already stacked sheets. It becomes easy to stick with the image. That is, even in the face-down mode, if the amount of toner of the image formed in the manual duplex mode is large, an image is formed on the image-formed side of the already stacked sheets and the manual duplex mode. Blocking phenomenon easily occurs between the two surfaces. Therefore, in the present embodiment, the fan is operated when the toner amount of the image formed in the manual duplex mode is large.

  By the way, in the present embodiment, the toner amount of the image formed image is determined by counting the number of dots to which the toner adheres in the image data by the video counter 101 and based on the video count value which is this count value. To do. Note that this video count value is the sum of 1 parts when the image data is represented by a data 1 part developed with toner and a data 0 part not developed, for example. When the first video count value A is less than the preset value a1, the blocking phenomenon does not occur. Therefore, even if the paper discharge sensor 88 detects the leading edge of the sheet S, the fan 86 is not operated. . Conversely, if the first video count value A is greater than or equal to the preset value a1, a blocking phenomenon is likely to occur. Therefore, when the paper discharge sensor 88 detects the leading edge of the sheet S, the fan is activated. When the amount of toner is large, the image forming surface side of the sheet tends to shrink due to the toner. The contraction direction is opposite to the curl generated in the sheet when passing through the pull-in path, and therefore the sheet is less likely to curl. Further, when the amount of toner is large, the weight of the sheet S also increases. For this reason, even if the fan is operated, the discharged sheet S does not float. Therefore, even if the fan is operated, the blocking phenomenon can be prevented without reducing the loadability.

  Next, the operation control of the fan in this embodiment will be described with reference to the flowchart shown in FIG. First, a case will be described in which an image is formed on the back side of a sheet on which an image is formed on one side, and then the sheet is discharged in a face-up mode in which a new image-formed surface is an upper surface on the discharge tray 65. In this case, when the image forming job is started, the CPU 89 recognizes whether the mode designated in advance by the operation unit or an external PC (not shown) is the face-down mode (S11). Here, when the face-up mode is designated, that is, when the designated mode is not the face-down mode (N in S11), the sheet S is subsequently conveyed, and when the paper discharge sensor 88 detects the leading edge of the sheet S (S17). ), The fan 86 is operated (S18). Next, it is determined whether the job is completed (S16). Until the job is completed (N in S16), the fan 86 continues to operate in the face-up mode for the second and subsequent sheets. When the job is finished (Y in S16), the fan 86 is stopped (S19).

  Next, a description will be given of a case where a sheet is discharged in the face-down mode in which the surface on which a new image is formed is the lower surface on the discharge tray 65. In this case, that is, in the face-down mode (Y in S11), it is determined whether or not the manual double-side mode is set (S12). Here, in the manual duplex mode (Y in S12), the video counter 101 as the toner amount detection unit detects the toner amount for image formation on the surface on which a new image is formed. When the first video count value A, which is detection information from the video counter 101, is less than the preset value a1 (Y in S13), even if the paper discharge sensor 88 detects the leading edge of the sheet S ( S14), the fan 86 does not operate (S15). Conversely, when the first video count value A is greater than or equal to the preset value a1 (N in S13), when the paper discharge sensor 88 detects the leading edge of the sheet S (S17), the fan 86 is selectively activated. (S18). When the second and subsequent sheets are also in face-down mode (Y in S11) and in manual duplex mode (Y in S12), the CPU 89 controls the operation and stop of the fan 86 according to the video count value A of the image to be formed. To do. At the end of the job, the fan 86 is stopped (S19). If the manual duplex mode is not set (N in S12), the fan 86 is stopped (S15) when the paper discharge sensor 88 detects the leading edge of the sheet S (S14), as in the first embodiment described above. .

  As described above, in the present embodiment, even in the face-down mode, when the manual double-side mode is selected and the toner amount of the image newly formed on the back surface exceeds the predetermined amount, the fan 86 is used. Is activated. As a result, blocking phenomenon (sheet sticking) can be prevented and sheet stackability can be maintained. In this embodiment, the control when the manual duplex mode is selected has been described. However, the present invention is also effective in the automatic duplex mode in which images are continuously formed on the front and back surfaces. When the automatic double-side mode is selected and reverse paper discharge is performed to align the page order, if the amount of toner of the image newly formed on the back surface exceeds a predetermined amount, the blocking phenomenon ( Sheet sticking) can be prevented.

DESCRIPTION OF SYMBOLS 1 ... Color laser printer, 1A ... Color laser printer main body, 1B ... Image formation part, 1F ... Outlet, 5 ... Fixing device, 65 ... Paper discharge tray, 77 ... Paper discharge roller, 82 ... Paper discharge conveyance path, 83 ... Pull-in path, 84 ... switchback path, 86 ... fan, 88 ... discharge sensor, 89 ... CPU (control unit), 90 ... reverse discharge path, 100 ... operation unit, 101 ... video counter, S ... sheet

Claims (3)

A fixing unit for fixing the toner image on the sheet;
A discharge unit for discharging the sheet on which the toner image is fixed to the sheet stacking unit;
A discharge passage for guiding the sheet on which the toner image is fixed to the sheet stacking unit;
An air outlet that is provided between the discharge unit and the sheet stacking unit, and blows out air between the sheet discharged by the discharge unit and the sheet already discharged to the sheet stacking unit;
Blower means for blowing air blown out from the blowout port toward the blowout port;
A controller that controls the blowing operation of the blowing means,
The control unit fixes the sheet on which the toner image is fixed on the back side opposite to the one side of the sheet on which the image is already formed on one side, when the back side is discharged toward the sheet stacking unit. The blower is operated when the toner amount of the toner image exceeds a predetermined amount, and the blower is not operated when the toner amount of the toner image fixed on the back surface is less than the predetermined amount. An image forming apparatus. A toner amount detection unit that obtains the toner amount of the toner image formed on the back surface by counting image data developed with toner;
The image forming apparatus according to claim 1, wherein the control unit controls the air blowing unit to operate based on detection information of the toner amount detection unit.   A pull-in path that branches from the discharge passage and temporarily pulls a sheet on which the toner image is fixed on the back surface, then reverses the front and back surfaces and the front and rear ends in the sheet conveyance direction, and guides the back surface toward the sheet stacking unit. The image forming apparatus according to claim 1, further comprising:

Images