Patent Publication Number: US-11385584-B2

Title: Image forming apparatus

Description:
BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This invention relates to an image forming apparatus forming an image on a sheet. 
     Description of the Related Art 
     In an image forming apparatus such as a printer and a copier, there is an apparatus which forms an image on a sheet by an electrophotographic system. In the electrophotographic system, after a toner image has been transferred to the sheet, the toner image is fixed on the sheet by being conveyed to a fixing unit which heats and presses the sheet. In a case where a narrow sheet width sheet is continuously conveyed to the fixing unit, because of heat transmission to the sheet, a so-called sheet non-passing portion temperature rise by which a temperature rise at a sheet non-passing portion becomes larger than the temperature rise at a sheet passing portion occurs at a fixing nip portion at which the sheet is heated and pressed. 
     The sheet non-passing portion temperature rise at the fixing nip portion causes unevenness in a film feed speed in a fixing unit of a film heating method described in Japanese Patent Laid-Open H04-204980. Further, since the sheet non-passing portion temperature rise also causes ununiform expansion of a pressing roller of the fixing unit, there is a problem of degradation of a printing quality, such as occurrence of wrinkles of the sheet and defective fixing of the toner, which needs to be tackled. 
     Hitherto, when the narrow sheet width sheet is conveyed to the fixing nip portion, as a countermeasure to the sheet non-passing portion temperature rise, control by which a sheet feed interval is widened so as to decrease temperature at the nip portion by extending a time during which the sheet does not exist at the nip portion is performed. However, depending on a configuration of the apparatus, sometimes, it is possible to judge the sheet width only in two sizes of a normal size and a small size. In this case, when it is judged as the small size, the sheet feed interval is set by assuming the minimum width sheet usable for the apparatus so as to prevent the degradation of the printing quality due to the sheet non-passing portion temperature rise. 
     Accordingly, in a case where the sheet width of the sheet actually used is small but relatively large, the sheet feed interval is unnecessarily widened so that productivity of printing is decreased. 
     SUMMARY OF THE INVENTION 
     According to one aspect of the present invention, an image forming apparatus includes an image forming unit configured to form a toner image on a sheet, a sheet conveyance unit configured to convey the sheet toward the image forming unit in a sheet conveyance direction, a fixing unit including a rotary member pair configured to form a nip portion and a heating element configured to heat the nip portion, the fixing unit being configured to fix the toner image on the sheet by heating the sheet on which the toner image is formed at the nip portion, a sheet width detection unit configured to output a first signal and a second signal, the sheet width detection unit being configured to output the first signal in a case where a sheet length in a width direction orthogonal to the sheet conveyance direction is equal to or larger than a first threshold value, the sheet width detection unit being configured to output the second signal in a case where the sheet length in the width direction is less than the first threshold value, a sheet length detection unit configured to output an output value corresponding to a sheet length in the sheet conveyance direction, and a control unit configured to perform a first mode, a second mode, and a third mode in a job continuously conveying a plurality of sheets, the first mode being a mode in which the sheet conveyance unit is controlled so that an arrival time from a passage of a trailing edge of a preceding sheet through the nip portion to an arrival of a leading edge of a succeeding sheet succeeding to the preceding sheet at the nip portion becomes a first time, the second mode being a mode in which the sheet conveyance unit is controlled so that the arrival time becomes a second time, the third mode being a mode in which the sheet conveyance unit is controlled so that the arrival time becomes a third time, the second time being longer than the first time, the third time being longer than the second time. The control unit is configured to perform the first mode in a case where the sheet width detection unit outputs the first signal, and configured to perform the third mode in a case where the sheet width detection unit outputs the second signal and the sheet length detection unit outputs the output value not corresponding to a length, in the sheet conveyance direction, of a sheet size information specified by the job, and configured to perform the second mode in a case where the sheet width detection unit outputs the second signal and the sheet length detection unit outputs the output value corresponding to the length, in the sheet conveyance direction, of the sheet size information. 
     Further features of the present invention will become apparent from the following description of exemplary embodiments with reference to the attached drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic view of a configuration of a printer according to a first embodiment. 
         FIG. 2  is a block diagram of a control configuration of the printer according to the first embodiment. 
         FIGS. 3A and 3B  are drawings showing a configuration of a setting screen according to the first embodiment. 
         FIG. 4  is a drawing illustrating an aspect of detection of a sheet size according to the first embodiment. 
         FIG. 5  is a schematic cross-sectional view of a fixing unit according to the first embodiment. 
         FIG. 6  is a flowchart showing a flow of a sheet conveyance operation control according to the first embodiment. 
         FIG. 7  is a diagram showing results of the sheet conveyance operation control according to the first embodiment. 
         FIGS. 8A and 8B  are flowcharts showing a flow of a sheet conveyance operation control according to an embodiment alternative to the first embodiment. 
         FIG. 9  is a diagram showing results of the sheet conveyance operation control according to the embodiment alternative to the first embodiment. 
         FIG. 10  is a flowchart showing a flow of a sheet conveyance operation control according to a second embodiment. 
     
    
    
     DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, exemplary embodiments of this disclosure will be described with reference to attached drawings. 
     First Embodiment 
     General Arrangement of Image Forming Apparatus 
     At first, with reference to  FIG. 1 , a configuration of a printer  1 , serving as an image forming apparatus in this embodiment, will be described.  FIG. 1  is a schematic configuration diagram of the printer  1 . The printer  1  includes a feed cassette  20 , an image forming engine  1 A of an electrophotographic system, a charge unit  33  for a remaining toner, a fixing unit  50 , and a control unit  304 . Based on an image forming job which is input from a connected external apparatus, an operation part, not shown, and the like, the printer  1  performs a printing process by forming toner images of yellow (Y), magenta (M), cyan (C), and black (K), and fixing the toner image on a sheet S. The image forming engine  1 A, serving as an image forming unit in this embodiment, includes photosensitive member drums  22 Y,  22 M,  22 C, and  22 K, charge units  23 Y,  23 M,  23 C, and  23 K, and toner cartridges  25 Y,  25 M,  25 C, and  25 K. Further, the image forming engine  1 A also includes developing units  26 Y,  26 M,  26 C, and  26 K, an intermediate transfer member  30 , primary transfer units  31 Y,  31 M,  31 C, and  31 K, and a secondary transfer roller  32 . To be noted, in the following descriptions, an operation to form the toner image of yellow (Y) will be described as an example, and, regarding the toner images of the other colors, duplicating descriptions will be omitted herein. 
     The photosensitive member drum  22 Y, whose outer peripheral surface of an aluminum cylinder is coated with an organic photo conductive layer, is rotatably driven counter-clockwise by a driving force of a drive motor, not shown. The charge unit  23 Y uniformly charges the surface of photosensitive member drum  22 Y. Based on the job, the control unit  304  controls a laser scanner  24 Y so that the surface of photosensitive member drum  22 Y is selectively exposed with an irradiated laser beam. Herewith, an electrostatic latent image to form an image based on the job is formed on the surface of photosensitive member drum  22 Y. The electrostatic latent image formed on the photosensitive member drum  22 Y is developed as the toner image of yellow by the developing unit  26 Y. To be noted, the developing units  26 Y,  26 M,  26 C, and  26 K are respectively capable of coming into contact with and being separated from the photosensitive member drums  22 Y,  22 M,  22 C, and  22 K by a contact/separation mechanism, not shown. 
     The intermediate transfer member  30  is an endless belt made of resin, and disposed in a state being in contact with the photosensitive member drums  22 Y,  22 M,  22 C, and  22 K. The intermediate transfer member  30  is rotatably driven clockwise by a driving force of a drive motor, not shown, along with rotation of the photosensitive member drums  22 Y,  22 M,  22 C, and  22 K. The toner image of yellow on the surface of the photosensitive member drum  22 Y is transferred to the intermediate transfer member  30  by being applied with a voltage by the primary transfer unit  31 Y. By a series of the processes as described above, the toner images of Y, M, C, and K are transferred to the intermediate transfer member  30  in sequence (primary transfer). Toners remaining on the photosensitive member drums  22 Y,  22 M,  22 C, and  22 K are respectively collected by cleaning units  27 Y,  27 M,  27 C, and  27 K. 
     The sheet S stacked on the feed cassette  20  is fed to the image forming engine  1 A by a feed roller  21  and a retard roller  28 , constituting a sheet feed unit. The sheet feed unit of this embodiment is constituted by the feed cassette  20 , the feed roller  21 , and the retard roller  28 . The sheet fed from the feed cassette  20  is conveyed by a registration roller pair  29  to a transfer nip portion formed by the secondary transfer roller  32  and an inner roller  34  in a timing synchronizing with the image formation at the image forming engine  1 A. The registration roller pair  29  is freely rotatably disposed between the feed roller  21  and the image forming engine  1 A with respect to a sheet conveyance direction. Further, the registration roller pair  29  is capable of correcting a skew of the sheet S by bringing the sheet S into contact with a nip portion of the registration roller pair  29  with the registration roller pair  29  stopping rotation. The control unit  304  judges a size of the sheet S based on detection results of a sheet length sensor  15  and a sheet width sensor  16  disposed downstream of the registration roller pair  29  in the sheet conveyance direction. A sheet length detection unit of this embodiment is the sheet length sensor  15 , and a sheet width detection unit is the sheet width sensor  16 . Thereafter, in a state where the sheet S is nipped by the secondary transfer roller  32  and the intermediate transfer member  30  which is brought into pressure contact with the secondary transfer roller  32  by the inner roller  34 , the voltage is applied to the secondary transfer roller  32 , and the toner image on the intermediate transfer member  30  is transferred to the sheet S (secondary transfer). The sheet S on which the toner image has been transferred is conveyed to the fixing unit  50 . To be noted, the charge unit  33  is for charging the toner remained on the intermediate transfer member  30  after the secondary transfer. The toner remained on the intermediate transfer member  30  is charged in the reverse of an original polarity by the charge unit  33 , collected electrostatically by the primary transfer unit  31  on the photosensitive member drum  22 , and finally removed by the cleaning unit  27 . 
     The fixing unit  50 , serving as a fixing unit of this embodiment, fixes the toner on the sheet S by pressing and heating the sheet S, and a detail configuration will be described later. The sheet S with the toner fixed is discharged to a sheet discharge tray  56  by sheet discharge rollers  54  and  55 , and the printing operation ends. In this embodiment, a sheet conveyance unit conveying the sheet is constituted by the feed roller  21 , the retard roller  28 , the registration roller pair  29 , the secondary transfer roller  32 , the intermediate transfer member  30 , and the sheet discharge rollers  54  and  55 . To be noted, in this embodiment, a process speed at the image forming engine  1 A is set at 132 mm/second, and a sheet feed interval from the feed cassette  20  is set at 24 ppm (pages per minute) in a case longitudinally feeding an A4 size. 
     Control Configuration of Image Forming Apparatus 
     Next, with reference to  FIG. 2 , a control configuration of the printer  1  will be described.  FIG. 2  is a function block diagram showing a configuration of the control unit  304  performing control of the printer  1 . The control unit  304  is constituted by including a CPU (central processing unit)  71 , serving as a calculation unit, a memory  72 , serving as a memory unit, including a ROM (read only memory) and a RAM (random access memory), an I/F (interface)  73  controlling communication with an external apparatus, and the like. The RAM stores information input to the control unit  304 , detected information, a calculation result of the CPU  71 , and the like. The ROM stores a control program of the printer  1 , a data table, and the like. It is possible to reciprocally transmit and read in data between the CPU  71  and the memory  72 , and the memory  72  is used as a work area when the CPU  71  performs a calculation. 
     Each unit of a mechanism which forms the image on the sheet by the image forming process described in  FIG. 1  is coupled to the control unit  304 . For example, a fixing control unit  320  performs temperature control and the like of the fixing unit  50 . Further, a feed/conveyance control unit  330  performs feed interval control of the sheet from the feed cassette  20 , and an image forming control unit  340  controls the process speed and the operations such as the developing, charging, and transferring. Further, an external apparatus  200 , such as a personal computer, communicably coupled to the printer  1  is coupled to the control unit  304 . 
     Job information (including such as a print start command, print mode information such as monochrome and color, setting information such as sheet size information, an image signal of a print object) is input to the control unit  304 . The control unit  304  controls to bring each unit of the printer  1  to execute an image forming operation forming the toner image on the sheet. To be noted, it is possible for a user to input the setting information and the print start command from a setting screen  210  (refer to  FIG. 3 ) displayed on a display part  200 A of the external apparatus  200  by a printer driver mounted in the external apparatus  200 , a setting screen displayed in an operation part, not shown, and the like. The job information transmitted to the printer  1  is input to the CPU  71  via the I/F  73  in the control unit  304 . 
     In this embodiment, a size selection button  211  is disposed in the setting screen  210 .  FIGS. 3A and 3B  indicate an example of a configuration of the setting screen  210 . To be noted, the setting screen  210  is a screen displayed in the external apparatus  200 , the operation part, not shown, and the like. When the size selection button  211  shown in  FIG. 3A  is selected, a pull-down menu as shown in  FIG. 3B  is displayed. The pull-down menu includes buttons, such as the A4 size  211 A, a LTR (letter) size  211 B, a B5 size  211 C, and an A5 size  211 D, to select the sheet size. When any of the sheet size (for example, the A4 size  211 A) is selected from the pull-down menu and an OK button  212  (refer to  FIG. 3A ) is tapped, it is reflected to the sheet size information that the sheet size of the sheet used for the printing is the A4 size. Then, when an operation to execute the print start command is performed, the job information including the sheet size information which indicates the sheet used for the printing is the A4 size is input to the control unit  304 . Herewith, the control unit  304  starts execution of the image forming operation based on the job information. 
     Aspect of Detection of Sheet Size 
     Next, with reference to  FIG. 4 , an aspect of detection of the sheet size in this embodiment will be described.  FIG. 4  is a diagram exemplifying arrangement positions of the sheet length sensor  15  and the sheet width sensor  16 . As shown in  FIG. 4 , in this embodiment, sheet width sensors  16 A and  16 B are disposed on both sides of the sheet across a conveyance reference position P 1 , and the sheet length sensor  15  is disposed at a position closer to the conveyance reference position P 1  than the sheet width sensor  16 B. Since configurations of the sheet width sensors  16 A and  16 B are common, in the following descriptions, both of the sheet width sensors  16 A and  16 B are mentioned as the sheet width sensor  16  when it is not necessary to distinguish from each other. In this embodiment, the sheet length sensor  15  and the sheet width sensor  16  are configured to include a lever member which is brought down when an edge of the sheet comes into contact, and ON and OFF signals are respectively output in a brought down state and not in the brought down state of the lever member. 
     At first, the aspect of the detection of a sheet length in the sheet conveyance direction by the sheet length sensor  15  will be described. When the job is started at the printer  1 , the sheet is fed from the feed cassette  20 . A conveyance speed of the sheet S is adjusted to synchronize with a timing of the image formation at the image forming engine  1 A by controlling, for example, the feed interval from the feed cassette  20  and the like. The lever member disposed at a detection position of the sheet length sensor  15  which is disposed downstream of the registration roller pair  29  in the sheet conveyance direction is brought down by a leading edge of the sheet S. Thereafter, the sheet length sensor  15  outputs the ON signal during a time until a trailing edge of the sheet S has passed through the detection position of the sheet length sensor  15 . Accordingly, in a case where the conveyance speed of the sheet S is kept constant, the sheet length sensor  15  outputs an output value corresponding to the length of the sheet S in the conveyance direction. The output value of the sheet length sensor  15  is input to the control unit  304 . The control unit  304  obtains the length of the sheet S in the sheet conveyance direction from duration of the ON signal of the sheet length sensor  15  and the conveyance speed of the sheet S (for example, the process speed at the image forming engine  1 A). Sheet throughput is maintained by starting a feed of a succeeding sheet in a timing when a predetermined time has passed after the leading edge or trailing edge of the proceeding sheet antecedently fed from the feed cassette  20  passed through the sheet length sensor  15 . Further, it is acceptable that the control unit  304  starts the feed of the succeeding sheet succeeding to the proceeding sheet fed from the feed cassette  20  corresponding to the length of the sheet S in the sheet conveyance direction. As described above, it is possible to perform a feedback control of an arrival time from when the trailing edge of the preceding sheet has passed through the nip portion N (refer to  FIG. 5 ) of the fixing unit  50  until when the leading edge of the succeeding sheet arrives at the nip portion N. Further, since the sheet length sensor  15  is disposed at the position closer to the conveyance reference position P 1  than the sheet width sensor  16 B in a width direction, it is possible to accurately detect the length of the sheet S in the sheet conveyance direction also in a case of a narrow width sheet. 
     Next, a length of the sheet in the width direction orthogonally intersecting with the sheet conveyance direction will be described. The lever members disposed at detection positions of the sheet width sensors  16 A and  16 B disposed downstream of the registration roller pair  29  in the sheet conveyance direction are brought down by the leading edge of the sheet S. As shown in  FIG. 4 , each of the sheet width sensors  16 A and  16 B is disposed with a distance of 95 mm from the conveyance reference position P 1  in the width direction. That is, a distance between the sheet width sensors  16 A and  16 B in the width direction is 190 mm. At this point, it is assumed that the sheet S is conveyed in a manner that a center of the sheet S in the width direction is along the conveyance reference position P 1 . In a case of a sheet S 1  whose length in the width direction is the A4 size (210 mm), the lever members disposed at the detection positions of the sheet width sensors  16 A and  16 B are brought down. On the other hand, in a case of a sheet S 2  whose length in the width direction is the B5 size (182 mm), since the sheet does not pass the detection positions of the sheet width sensors  16 A and  16 B, the lever members of the sheet width sensors  16 A and  16 B are not brought down. That is, the sheet width sensor  16  outputs the ON signal, which is a first signal, in a case where the sheet length in the width direction is equal to or larger than the distance between the sheet width sensors  16 A and  16 B. On the other hand, the sheet width sensor  16  outputs the OFF signal, which is a second signal, in a case where the sheet length in the width direction is less than the distance between the sheet width sensors  16 A and  16 B. A first threshold value of this embodiment is the distance between the sheet width sensors  16 A and  16 B in the width direction. As described above, the sheet width sensor  16  outputs the signal depending on whether the sheet length in the width direction is equal to or larger than the distance between the sheet width sensors  16 A and  16 B, that is, equal to or larger than the first threshold value, or less than the distance, that is, less than the first threshold value. The output value of the sheet width sensor  16  is input to the control unit  304 . In a case of an example shown in  FIG. 4 , the distance between the sheet width sensors  16 A and  16 B, which is the first threshold value, is 190 mm, and the control unit  304  recognizes the sheet with the sheet length less than 190 mm in the width direction as a small size sheet. 
     Configuration of Fixing Unit 
     Next, with reference to  FIG. 5 , a configuration of the fixing unit  50  of this embodiment will be described.  FIG. 5  is a schematic cross-sectional view of the fixing unit  50 . The fixing unit  50  includes a film unit  51  and a pressing roller  52 . The film unit  51  is constituted by including a fixing film  64 , serving as a tubular rotary member, a heater  63 , serving as a heating element, and a heater holder  65  supporting the heater  63 . Further, the pressing roller  52  constituting a rotary member pair with the fixing film  64  is an elastic rotary member, and is disposed at a position facing the film unit  51 . The fixing film  64  and the pressing roller  52  form the nip portion N through which the sheet S passes at the fixing unit  50 . A toner T carried on the sheet S conveyed to the nip portion N is heated by the heater  63  via the fixing film  64 , and pressed by the pressing roller  52 . Herewith, the toner T is fixed on the sheet S. 
     The heater  63  is constituted by a resistance heating element of silver palladium alloy and the like disposed on a ceramic board of alumina and the like, and the resistance heating element is coated with an over coat glass so that insulation and wear resistance properties of the resistance heating element are improved. The heater  63  is disposed inside the fixing film  64 , and a layer of the over coat glass comes into contact with an inner peripheral surface of the fixing film  64 . To be noted, so as to improve lubricity with the fixing film  64 , a small quantity of lubricant such as heat resistance grease is coated on a surface of the heater  63 . On a surface opposite a sliding surface, on which the heater  63  and the fixing film  64  come into contact with each other, of the heater  63 , a thermistor  66  is disposed. Based on a detection signal of the thermistor  66 , the control unit  304  controls, via the fixing control unit  320  (refer to  FIG. 2 ), an electrical current energized to the heater  63  so that the heater  63  is brought to a desired target temperature. 
     The fixing film  64  is formed as a composite layer film and includes a base layer which is cylindrically formed by a metal thin tube blank of such as stainless steel and kneaded matter of a heat resistance resin such as polyimide and a thermally conductive filler such as graphite and a releasing layer coating or tube-covering a surface of the base layer, directly or via a primer layer. The releasing layer is formed of PFA (p-fluorophenylalanine), PTFE (polytetrafluoroethylene), FEP (fluorinated ethylene-propylene copolymer), and the like. The fixing film  64  used in this embodiment is coated with PFA on the surface of the base layer formed by polyimide, and a total film thickness and a circumferential length are respectively 70 μm and 57 mm. The pressing roller  52  consists of an elastic layer  61 , which is formed by foaming a heat resistance rubber, such as a silicon rubber and a fluoro rubber having an insulating property, on a core metal  60  made of iron and the like, and the elastic layer is coated with a RTV (room temperature vulcanizing) silicone rubber having an adhesive property. Further, the pressing roller  52  consists of a releasing layer  62  which is formed by covering, or by applying a coating method, with a tube dispersing a conductive agent such as carbon in PFA, PTFE, FEP, and the like. In this embodiment, the silicon rubber is used for the elastic layer  61 , an outer diameter and a roller hardness of the pressing roller  52  are respectively 18 mm and 48° (Asker-C, weight load 600 g). The pressing roller  52  is pressed by a pressing member, not shown, with pressure of 180 N (newtons) so as to form the nip portion N extending between both edges in an axial direction with the fixing film  64  in between. Further, the pressing roller  52  is rotatably driven from the edge in the axial direction by a rotational driving unit, not shown, in an arrow direction in  FIG. 5  (counter-clockwise direction) via the core metal  60 . Herewith, the fixing film  64  is rotatably driven in an arrow direction in  FIG. 5  (clockwise direction) around an outside of the heater holder  65 . 
     The heater holder  65  is formed by a liquid crystal polymer, a phenol resin, PPS (polyphenylene sulfide), PEEK (polyetheretherketone), and the like, and supports the heater  63 . The fixing film  64  is externally fitted to the heater holder  65  with a margin, and disposed freely rotatably. The sheet S passes through the nip portion N formed between the pressing roller  52  and the fixing film  64 . Heat supplied from the heater  63  heats the sheet S at the nip portion N via the fixing film  64 . An unfixed toner T carried on the sheet S is melted by the heat received from the heated fixing film  64  and the pressure at the nip portion N, and fixed on the sheet S. The fixing control unit  320  includes a temperature control program to control a temperature of the heater  63 , and the control unit  304  performs control so that the temperature of the heater  63  is brought to the desired target temperature based on a detected temperature of the thermistor  66 . As a control method, a PID (proportional integral and differential) control consisting of a proportional term, an integral term, and a differential term is preferred. An energization time of the heater  63  in a cycle is determined by the PID control, and an output power to the heater  63  is determined by driving a heater energization time control circuit, not shown. In this embodiment, the output power to the heater  63  is updated in every 100 milliseconds as a control cycle. 
     Relationship Between Sheet Non-passing Portion Temperature Rise and Throughput 
     In the fixing unit  50 , a length of a resistance heating layer of the heater  63  in the axial direction of the fixing film  64  is set at a length at which a fixability of the toner to the sheet at edges of the maximum size sheet usable by the printer  1  is ensured. On the other hand, in a case where the sheet with a small length in the width direction (narrow width sheet) passes through the nip portion N, it occurs that the heat is not transmitted to the sheet, and accumulated to cause a high temperature in each of the heater  63 , the fixing film  64 , the pressing roller  52 , and the like. A phenomenon in which a temperature rise at a sheet non-passing portion in the nip portion N (sheet non-passing portion) becomes larger than a temperature rise at a sheet passing portion is referred to as a sheet non-passing portion temperature rise. Regarding the sheet non-passing portion temperature rise, the shorter the sheet length in the width direction is, the larger a difference in heat consumption between the sheet passing portion and the sheet non-passing portion and the larger the temperature rise at the sheet non-passing portion become. When an extent of the sheet non-passing portion temperature rise becomes larger, since the pressing roller  52  of the pressing roller  52  disproportionally expands with the heat and a feed speed of the fixing film  64  deviates, a problem such as wrinkles on the sheet and defective fixing of the toner occurs. 
     In a conventional printer, so as to prevent the problem caused by the sheet non-passing portion temperature rise, as shown in an operation in a reference example, in a case where the sheet passing through the nip portion N is the small size sheet, control to lengthen a feed interval of the sheet, so-called control to decrease the throughput, is performed. At this point, a sheet conveyance operation in the reference example will be described. Based on the input job information, the control unit  304  controls to bring each unit of the printer  1  to execute the image forming operation. When the image forming engine  1 A starts the image formation at the predetermined process speed, the feed of the sheet from the feed cassette  20  is started. When the sheet passes through the registration roller pair  29 , the leading edge of the sheet reaches the detection position of the sheet width sensor  16 . At this time, depending on whether or not the lever member of the sheet width sensor  16  is brought down, the control unit  304  judges whether the sheet fed from the feed cassette  20  is a normal size or the small size. 
     In the example shown in  FIG. 4 , when the lever member of the sheet width sensor  16  is not brought down and it is identified that the sheet is the small size sheet, it is known that the sheet length in the width direction is less than 190 mm, but an exact sheet length in the width direction is unknown. Therefore, there is not enough evidence to judge whether or not the sheet specified by the user in the sheet size information included in the job information is the sheet actually used in the image forming operation. Further, since the sheet non-passing portion temperature rise occurs during the passage of the sheet through the nip portion N, it is possible to reduce the temperature rise by extending a time (sheet gap) of the sheet not existing in the nip portion N. However, in a case where the sheet length in the width direction is judged based on the output values of two sensors, such as the sheet width sensors  16 A and  16 B, whether or not to extend the sheet gap is judged in two cases of whether the sheet is the normal size (for example, A4 size) or the small size. At this point, in a case where it is judged to be the small size sheet, so as to prevent the problem from the sheet non-passing portion temperature rise, it is necessary to set the sheet gap on the assumption that the sheet length in the width direction is the minimum sheet length at which the extent of the sheet non-passing portion temperature rise is maximized. At this point, as the minimum sheet length in the width direction, 105 mm which is the sheet length in the width direction of a Com #10 size (105 mm×241 mm) is assumed. Therefore, in a case where an actually used sheet is the small size sheet but the sheet length in the width direction is relatively large (for example, 182 mm, equivalent to the B5 size), the sheet gap is widened more than necessary, and printing productivity is decreased. 
     Sheet Conveyance Operation 
     As described above, by the sheet conveyance operation of the reference example, it is possible to suppress an effect of the sheet non-passing portion temperature rise, but, on the other hand, it involves a problem that causes damage to the printing productivity. On the other hand, by a sheet conveyance operation control of this embodiment, the sheet feed interval from the feed cassette  20  is changed based on the output value of the sheet length sensor  15 . With reference to  FIG. 6 , a flow of the sheet conveyance operation control of this embodiment will be described.  FIG. 6  is a flowchart showing the flow of the sheet conveyance operation control. In  FIG. 6 , descriptions are provided on the assumption that the sheet with a sheet size equal to or smaller than the B5 size is the small size sheet. Further, the flowchart shown in  FIG. 6  is mainly performed by the control unit  304 . Based on the input job information, the control unit  304  controls to bring each unit of the printer  1  to execute the image forming operation (STEP S 101 ). At this time, the control unit  304  obtains the sheet size information which is instructed in the job information continuously conveying a plurality of sheets (STEP S 102 ). It is possible to separate the sheet size information into information on the sheet length in the sheet conveyance direction (hereinafter referred to as sheet length information) and information on the sheet length orthogonally intersecting with the sheet conveyance direction (hereinafter referred to as sheet width information). At this point, the sheet length information and the sheet width information of the B5 size are respectively 257 mm and 182 mm. 
     When the image forming engine  1 A starts the image formation with the predetermined process speed, the sheet conveyance from the feed cassette  20  is started. When the sheet passes through the registration roller pair  29 , the leading edge of the sheet reaches the detection position of the sheet width sensor  16 . At this time, in a case where the lever member of the sheet width sensor  16  is brought down and the ON signal is output, it is judged that the fed sheet is not the small size sheet (STEP S 103 : NO). In a case where the fed sheet is not the small size sheet, the control unit  304  performs the feed and conveyance of the sheet by setting the sheet feed interval at the feed interval suitable for the normal size sheet (for example, the feed interval suitable for the A4 size sheet is a fourth time) (STEP S 106 ). 
     On the other hand, in a case where the lever member of the sheet width sensor  16  is not brought down and the OFF signal is output, it is judged that the fed sheet is the small size sheet (STEP S 103 : YES). In a case where the fed sheet is the small size sheet, the control unit  304  determines the sheet length in the sheet conveyance direction based on the sheet conveyance speed and the output value of the sheet length sensor  15  (STEP S 104 ). At this point, the sheet conveyance speed is, for example, the speed equivalent to the process speed of the image forming engine  1 A. Then, it is judged whether or not the sheet length in the conveyance direction determined at STEP S 104  is a length corresponding to the sheet length information of the sheet size information (STEP S 105 ). To be noted, in this embodiment, taking into consideration a detection error of the sheet length sensor  15 , within 10 mm of the length in the sheet length information of the sheet size information is regarded as corresponding to the length of the sheet length information in the sheet conveyance direction. In a case where the output value of the sheet length sensor  15  is an output value corresponding to the length of the sheet length information in the sheet conveyance direction (STEP S 105 : YES), it is judged that the length of the fed sheet corresponds to the length (B5 size) of the sheet length information of the sheet size information. Then, by setting the sheet feed interval at the B5 size (for example, a fifth time which is a feed interval longer than the feed interval of the A4 size) (STEP S 107 ), the feed and the conveyance of the sheet are performed. On the other hand, in a case where the output value of the sheet length sensor  15  is not the output value corresponding to the length of the sheet length information in the sheet conveyance direction (STEP S 105 : NO), it is judged that the length of the fed sheet does not correspond to the length of the sheet length information of the sheet size information. Then, by setting the sheet feed interval at a sixth time (STEP S 108 ) which is a feed interval longer than the feed interval of the B5 size, the feed and the conveyance of the sheet are performed. 
     As described above, in this embodiment, in a case where, based on the output value of the sheet width sensor  16 , it is judged that the small size sheet is fed, the length of the fed sheet in the sheet conveyance direction is judged based on the output value of the sheet length sensor  15 . Then, in a case where the judged sheet length in the sheet conveyance direction does not correspond to the length of the sheet size information specified by the job, the sheet feed interval is set at longer than the feed interval of the sheet size specified by the sheet size information. By performing the control as described above, an arrival time between the passage of the trailing edge of the preceding sheet antecedently conveyed to the nip portion N through the nip portion N and an arrival of the leading edge of the succeeding sheet succeeding to the preceding sheet at the nip portion N is changed. In particular, in the case where the lever member of the sheet width sensor  16  is brought down and the ON signal is output, a first mode by which the feed and the conveyance of the sheet from the feed cassette  20  is controlled so that the arrival time becomes a first time (an arrival time suitable for the A4 size) is performed. Further, in the case where the lever member of the sheet width sensor  16  is not brought down and the OFF signal is output, following two modes of controls are performed. In the case where the output value of the sheet length sensor  15  corresponds to the sheet length information, a second mode in which the feed and the conveyance of the sheet from the feed cassette  20  are controlled so that the arrival time becomes a time (second time) longer than the arrival time suitable for the A4 size is performed. On the other hand, in the case where the output value of the sheet length sensor  15  does not correspond to the sheet length information, a third mode in which the feed and the conveyance of the sheet from the feed cassette  20  are controlled so that the arrival time becomes a third time longer than the second time is performed. As described above, corresponding to the output values of the sheet length sensor  15  and the sheet width sensor  16 , the printer  1  is capable of performing the first, second, and third modes. Accordingly, in the case where the small size sheet is fed, the feed and the conveyance of the sheet are performed at an interval suitable for the sheet size specified by the sheet size information in the case where the sheet length of the small size sheet corresponds to the length in the sheet conveyance direction of the sheet size information specified by the job. 
     To be noted, it is acceptable to judge through the job whether or not the sheet length in the sheet conveyance direction corresponds to the sheet length information of the sheet size information. In this case, in a case where the length has changed from a corresponding length to a not corresponding length to the sheet length information during an execution of the job, it is suitable to perform the job by switching the mode from the second mode to the third mode. 
       FIG. 7  is a diagram showing a comparison result of throughputs of the sheet conveyance operation control of  FIG. 6  and the control of the reference example. In comparing the throughputs, it is assumed that the image formed on the sheet is a text image (not shown) with a coverage rate of 4%, and, in the setting screen  210  (refer to  FIG. 3 ), the B5 size  211 C, a single side, a full color, and 100 (copies) are respectively set for the sheet size, single side/duplex, a mode, and a number of copies. Further, it is assumed that 100 sheets of the B5 size sheet are set in the feed cassette  20  and the image forming operation is started by tapping the OK button  212 . As the B5 size sheet, CS-068 (trade name of Cannon Marketing Japan Inc.) with a grammage of 68 g/m2 is used. In  FIG. 7 , a horizontal axis and a vertical axis respectively show a number of continuously fed sheets and the throughput (sheet feed interval) determined between the Nth sheet and N+1th sheet of the number of continuously fed sheets, and a solid line α and a broken line β respectively show the result of the sheet conveyance operation control of  FIG. 6  and the result of the sheet conveyance operation control of the reference example. 
     From  FIG. 3 , although both of the solid line α and the broken line β declines to 7 ppm before the 10th sheet of the number of continuously fed sheets, the throughput of the solid line α does not decline in subsequent feeds. On the other hand, since, in a case of the broken line β, the feed interval of the sheet is widened assuming the Com #10 size sheet narrower than the B5 size in the width direction, the throughput declines on and after the 10th sheet, and falls to 2 ppm at the 70th sheet. In this embodiment, in the case where the small size sheet is fed, when the length of the small size sheet corresponds to the length of the sheet size information specified by the job in the sheet conveyance direction, the feed and the conveyance of the sheet are performed at the interval suitable for the sheet size specified by the sheet size information. By performing the control as described above, in this embodiment, it is possible to suppress the effect of the sheet non-passing portion temperature rise, and, since the sheet gap is not widened more than necessary, it is possible to improve the printing productivity. 
     Alternative Embodiment 
     In the first embodiment, the sheet length in the sheet conveyance direction is determined by defining singly the length in the sheet conveyance direction corresponding to the sheet size information specified by the job. In an alternative embodiment of the first embodiment, the feed and the conveyance of the sheet are controlled with the interval suitable for the length of the actually fed sheet in the sheet conveyance direction by defining the sheet sizes specified by the sheet size information (B5 size and A5 size) doubly. 
     With reference to  FIGS. 8A and 8B , a flow of the sheet conveyance operation control of this alternative embodiment will be described.  FIGS. 8A and 8B  are flowcharts showing the flow of the sheet conveyance operation control. In  FIGS. 8A and 8B , descriptions are provided on the assumption that a sheet with equal to or smaller than the B5 size is the small size sheet. Further, the flowcharts of  FIGS. 8A and 8B  are mainly performed by the control unit  304 . In the flowcharts of  FIGS. 8A and 8B , the same step as the flowchart of  FIG. 6  is put with the same mark, and duplicating descriptions will be omitted herein. Based on the input job information, the control unit  304  controls to bring each unit of the printer  1  to execute the image forming operation (STEP S 101 ). At this time, the control unit  304  obtains the sheet size information which is specified in the job information continuously conveying a plurality of sheets (STEP S 102 ). It is possible to separate the sheet size information into information on the sheet length in the sheet conveyance direction (hereinafter referred to as sheet length information) and information on the sheet length orthogonally intersecting with the sheet conveyance direction (hereinafter referred to as sheet width information). At this point, the sheet length information and the sheet width information of the B5 size are respectively 257 mm and 182 mm. Further, the sheet length information and the sheet width information of the A5 size sheet are respectively 210 mm and 148 mm. When the B5 size is specified by the sheet size information, the control unit  304  proceeds to STEP S 103 , and, when the A5 size is specified, the control unit  304  proceeds to STEP S 203 . To be noted, since, when the B5 size is specified by the sheet size information, steps subsequent to STEP S 103  is similar to the flowchart of  FIG. 6 , duplicating descriptions will be omitted herein, and steps subsequent to STEP S 103 , when the A5 size is specified by the sheet size information, will be described. 
     When the image forming engine  1 A starts the image formation with the predetermined process speed, the sheet conveyance from the feed cassette  20  is started. When the sheet passes through the registration roller pair  29 , the leading edge of the sheet reaches the detection position of the sheet width sensor  16 . At this time, in a case where the lever member of the sheet width sensor  16  is brought down and the ON signal is output, it is judged that the fed sheet is not the small size sheet (STEP S 203 : NO). In a case where the fed sheet is not the small size sheet, the control unit  304  performs the feed and conveyance of the sheet by setting the sheet feed interval at the normal size (for example, the feed interval suitable for the A4 size sheet) (STEP S 206 ). 
     On the other hand, in a case where the lever member of the sheet width sensor  16  is not brought down and the OFF signal is output, it is judged that the fed sheet is the small size sheet (STEP S 203 : YES). In a case where the fed sheet is the small size sheet, the control unit  304  determines the sheet length in the sheet conveyance direction based on the sheet conveyance speed and the output value of the sheet length sensor  15  (STEP S 204 ). At this point, the sheet conveyance speed is, for example, the speed equivalent to the process speed of the image forming engine  1 A. Then, it is judged whether or not the sheet length in the conveyance direction determined at STEP S 104  is a length corresponding to the sheet length information of the sheet size information (STEP S 205 ). To be noted, in this embodiment, taking into consideration a detection error of the sheet length sensor  15 , within 10 mm of the length in the sheet length information of the sheet size information is regarded as corresponding to the length of the sheet length information in the sheet conveyance direction. In a case where the output value of the sheet length sensor  15  is an output value corresponding to the length of the sheet length information in the sheet conveyance direction (STEP S 205 : YES), it is judged that the length of the fed sheet corresponds to the length (A5 size) of the sheet length information of the sheet size information. Then, by setting the sheet feed interval at the A5 size (for example, a feed interval longer than the feed interval of the A4 size) (STEP S 207 ), the feed and the conveyance of the sheet are performed. On the other hand, in a case where the output value of the sheet length sensor  15  is not the output value corresponding to the length of the sheet length information in the sheet conveyance direction (STEP S 205 : NO), it is judged that the length of the fed sheet does not correspond to the length of the sheet length information of the sheet size information. Then, by setting the sheet feed interval at a feed interval longer than the feed interval of the A5 size (STEP S 208 ), the feed and the conveyance of the sheet are performed. 
       FIG. 9  is a diagram showing a comparison result of throughputs of the sheet conveyance operation control of  FIG. 6  and the sheet conveyance operation control of  FIGS. 8A and 8B . In comparing the throughputs, it is assumed that the image formed on the sheet is a text image (not shown) with a coverage rate of 4%, and, in the setting screen  210  (refer to  FIG. 3 ), the A5 size  211 D, a single side, a full color, and 100 (copies) are respectively set for the sheet size, single side/duplex, a mode, and a number of copies. Further, it is assumed that 100 sheets of the A5 size sheet are set in the feed cassette  20  and the image forming operation is started by tapping the OK button  212 . As the A5 size sheet, PB PAPER A5 (trade name of Cannon Marketing Japan Inc.) with a grammage of 64 g/m2 is used. In  FIG. 9 , a horizontal axis and a vertical axis respectively show a number of continuously fed sheets and the throughput (sheet feed interval) determined between the Nth sheet and N+1th sheet of the number of continuously fed sheets, and a solid line α and a broken line β respectively show the result of the sheet conveyance operation control of  FIGS. 8A and 8B  and the result of the sheet conveyance operation control of  FIG. 6 . 
     From  FIG. 9 , although both of the solid line α and the broken line β declines to 5 ppm before the 10th sheet of the number of continuously fed sheets, the throughput of the solid line α does not decline in subsequent feeds. On the other hand, since the feed interval of the sheet is widened assuming the Com #10 size sheet is narrower than the A5 size in the width direction in a case of the broken line β, the throughput declines on and after the 10th sheet, and falls to 2 ppm at the 70th sheet. By performing the control as described above, in this alternative embodiment, it is also possible to suppress the effect of the sheet non-passing portion temperature rise, and, since the sheet gap is not widened more than necessary, it is possible to improve the printing productivity. To be noted, it is acceptable to specify equal to more than 2 sheet sizes in the sheet size information, and change the sheet feed interval by defining a business card size and a postcard size as the sheet size other than the A5 and B5 sizes. 
     Second Embodiment 
     In the first embodiment, the sheet length in the sheet conveyance direction is determined by defining singly the length in the sheet conveyance direction corresponding to the sheet size information specified by the job. Incidentally, sheet sizes usable for the printer  1  include a sheet size called an EXE size with the sheet length of 184 mm in the width direction and the sheet length of 267 mm in the sheet conveyance direction. In a case where the EXE size sheet is fed in the job, by the flowchart shown in  FIG. 6 , it sometimes occurs that the feed and the conveyance of the sheet are performed by widening the sheet feed interval longer than the feed interval of the B5 size. Since there are a little differences in sizes between the EXE size (184 mm×267 mm) and the B5 size (182 mm×257 mm), it is assumed that there are little differences in the effects of the sheet non-passing portion temperature rise. Therefore, in this embodiment, a determination threshold value to judge the sheet length in the sheet conveyance direction is increased. Herewith, it is intended to achieve both the suppression of the effect of the sheet non-passing portion temperature rise and the improvement of the printing productivity also in a case where the image formation is performed by using a sheet with a size similar to the size of the sheet specified by the sheet size information. 
     With reference to  FIG. 10 , a flow of the sheet conveyance operation control of this embodiment will be described.  FIG. 10  is a flowchart showing the flow of the sheet conveyance operation control. In  FIG. 10 , descriptions are provided on the assumption that a sheet with a size equal to or smaller than the B5 size is the small size sheet. Further, the flowchart of  FIG. 10  is mainly performed by the control unit  304 . Based on the input job information, the control unit  304  controls to bring each unit of the printer  1  to execute the image forming operation (STEP S 301 ). At this time, the control unit  304  obtains the sheet size information which is specified in the job information continuously conveying a plurality of sheets (STEP S 302 ). It is possible to separate the sheet size information into information on the sheet length in the sheet conveyance direction (hereinafter referred to as sheet length information) and information on the sheet length orthogonally intersecting with the sheet conveyance direction (hereinafter referred to as sheet width information). At this point, the sheet length information and the sheet width information of the B5 size are respectively 257 mm and 182 mm. Further, the sheet length information and the sheet width information of the EXE size are respectively 267 mm and 184 mm. 
     When the image forming engine  1 A starts the image formation with the predetermined process speed, the sheet conveyance from the feed cassette  20  is started. When the sheet passes through the registration roller pair  29 , the leading edge of the sheet reaches the detection position of the sheet width sensor  16 . At this time, in a case where the lever member of the sheet width sensor  16  is brought down and the ON signal is output, it is judged that the fed sheet is not the small size sheet (STEP S 303 : NO). In a case where the fed sheet is not the small size sheet, the control unit  304  performs the feed and conveyance of the sheet by setting the sheet feed interval at the feed interval suitable for the normal size sheet (for example, the feed interval suitable for the A4 size sheet) (STEP S 306 ). 
     On the other hand, in a case where the lever member of the sheet width sensor  16  is not brought down and the OFF signal is output, it is judged that the fed sheet is the small size sheet (STEP S 303 : YES). To be noted, since the sheet width sensors  16 A and  16 B are disposed at the distance of 190 mm, in a case where the fed sheet is a sheet with the EXE size, it is also judged that the fed sheet is the small size sheet. In a case where the fed sheet is the small size sheet, the control unit  304  determines the sheet length in the sheet conveyance direction based on the sheet conveyance speed and the output value of the sheet length sensor  15  (STEP S 304 ). At this point, the sheet conveyance speed is, for example, the speed equivalent to the process speed of the image forming engine  1 A. Then, it is judged whether or not the sheet length in the sheet conveyance direction determined at STEP S 304  is a length corresponding to the sheet length information of the sheet size information (STEP S 305 ). 
     To be noted, in this embodiment, taking into consideration the detection error of the sheet length sensor  15 , within 20 mm of the length in the sheet length information of the sheet size information is regarded as corresponding to the length of the sheet length information in the sheet conveyance direction. Herewith, it becomes possible to judge whether or not the sheet length in the sheet conveyance direction is a length corresponding to the B5 and EXE size sheets. In a case where the output value of the sheet length sensor  15  is an output value corresponding to the length of the sheet length information in the sheet conveyance direction (STEP S 305 : YES), it is judged that the length of the fed sheet corresponds to the length (B5 or EXE size) of the sheet length information of the sheet size information. Then, by setting the sheet feed interval at the B5 size or the EXE size (for example, a feed interval longer than the feed interval of the A4 size) (S 307 ), the feed and the conveyance of the sheet are performed. On the other hand, in a case where the output value of the sheet length sensor  15  is not the output value corresponding to the length of the sheet length information in the sheet conveyance direction (STEP S 305 : NO), it is judged that the length of the fed sheet does not correspond to the length of the sheet length information of the sheet size information. Then, by setting the sheet feed interval at a feed interval longer than the feed interval of the B5 or EXE size (STEP S 308 ), the feed and the conveyance of the sheet are performed. 
     As described above, in this embodiment, an arrival time between the passage of the trailing edge of the preceding sheet antecedently conveyed to the nip portion N through the nip portion N and an arrival of the leading edge of the succeeding sheet succeeding to the preceding sheet at the nip portion N is changed. In particular, in the case where the lever member of the sheet width sensor  16  is brought down and the ON signal is output, a first mode in which the feed and the conveyance of the sheet from the feed cassette  20  are controlled so that the arrival time becomes a first time (an arrival time suitable for the A4 size) is performed. Further, in the case where the lever member of the sheet width sensor  16  is not brought down and the OFF signal is output, following two modes of controls are performed. In the case where the output value of the sheet length sensor  15  corresponds to the sheet length information (B5 or EXE size), a second mode in which the feed and the conveyance of the sheet from the feed cassette  20  are controlled so that the arrival time becomes a time (second time) longer than the arrival time suitable for the A4 size is performed. On the other hand, in the case where the output value of the sheet length sensor  15  does not correspond to the sheet length information (B5 or EXE size), a third mode in which the feed and the conveyance of the sheet from the feed cassette  20  are controlled so that the arrival time becomes a third time which is longer than the second time is performed. 
     Accordingly, in the case where the small size sheet is fed, the feed and the conveyance of the sheet are performed at an interval suitable for the sheet size specified by the sheet size information in the case where the sheet length of the small size sheet corresponds to the length in the sheet conveyance direction of the sheet size information specified by the job. Further, also in a case where the size of the small size sheet is similar to the size of the sheet size information specified by the job, the feed and the conveyance of the sheet are performed at an interval suitable for the size specified by the sheet size information. Herewith, it is possible to achieve the suppression of the effect of the sheet non-passing portion temperature rise, and, since the sheet gap is not widened more than necessary, it is possible to improve the printing productivity. Further, also in a case where a sheet with a size similar to the size of the sheet specified by the sheet size information is fed, it is possible to achieve both the suppression of the effect of the sheet non-passing portion temperature rise and the improvement of the printing productivity. 
     Other Embodiments 
     Embodiment(s) of the present invention can also be realized by a computer of a system or apparatus that reads out and executes computer executable instructions (e.g., one or more programs) recorded on a storage medium (which may also be referred to more fully as a ‘non-transitory computer-readable storage medium’) to perform the functions of one or more of the above-described embodiment(s) and/or that includes one or more circuits (e.g., application specific integrated circuit (ASIC)) for performing the functions of one or more of the above-described embodiment(s), and by a method performed by the computer of the system or apparatus by, for example, reading out and executing the computer executable instructions from the storage medium to perform the functions of one or more of the above-described embodiment(s) and/or controlling the one or more circuits to perform the functions of one or more of the above-described embodiment(s). The computer may comprise one or more processors (e.g., central processing unit (CPU), micro processing unit (MPU)) and may include a network of separate computers or separate processors to read out and execute the computer executable instructions. The computer executable instructions may be provided to the computer, for example, from a network or the storage medium. The storage medium may include, for example, one or more of a hard disk, a random-access memory (RAM), a read only memory (ROM), a storage of distributed computing systems, an optical disk (such as a compact disc (CD), digital versatile disc (DVD), or Blu-ray Disc (BD)™), a flash memory device, a memory card, and the like. 
     While the present invention has been described with reference to exemplary embodiments, it is to be understood that the invention is not limited to the disclosed exemplary embodiments. The scope of the following claims is to be accorded the broadest interpretation so as to encompass all such modifications and equivalent structures and functions. 
     This application claims the benefit of Japanese Patent Application No. 2020-108953, filed Jun. 24, 2020, which is hereby incorporated by reference herein in its entirety.