Patent Publication Number: US-8121502-B2

Title: Fixing unit including heating regulator to adjust a heating width of a heating member

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This patent application is based on and claims priority pursuant to 35 U.S.C. §119 from Japanese Patent Application No. 2008-097368 filed on Apr. 3, 2008 in the Japan Patent Office, the entire contents of which are hereby incorporated herein by reference. 
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     Exemplary aspects of the present invention generally relate to an image forming apparatus, such as a copier, a facsimile machine, a printer, or a multi-functional system including a combination of at least two of these functions. 
     2. Description of the Background Art 
     Conventionally, a generally known image forming apparatus, such as a copier, a facsimile machine, a printer, or a multi-functional system including a combination of at least two of these functions, can form an image on a sheet serving as a printable recording medium in various sizes, for example, A4, A3, A5, B4, and so forth. To do so, the image forming apparatus employs a fixing unit to fix the image on the sheet with heat and pressure. The fixing unit, which extends across the entire width of that recording medium which has the widest width that the image forming apparatus can accommodate, includes a heating member, a fixing member, and an auxiliary fixing member. 
     With different sizes of the recording sheet, such as A4 size and A5 size, which are not as wide as the A3 size (297 mm) sheets having the widest width that the image forming apparatus can accommodate and therefore do not extend across the entire width of the fixing unit, when the fixing unit fixes the image onto the A4 size or A5 size sheet, temperatures of the fixing unit at or near lateral end portions of the heating member, the fixing member, and the auxiliary fixing member over which the A4 size and A5 size recording media do not extend rise significantly, resulting in problems such as uneven fixation of the image, deterioration of the fixing member, and so forth, ultimately causing damage to the fixing unit. 
     In particular, in order to reduce a rise time of the image forming apparatus, that is, the time required for the image forming apparatus to start printing after power is turned on, a high-power heating member and a relatively thin-film fixing member have come to be required. However, a drawback to this configuration is that temperatures at the end portions of the heating member, the fixing member, and so forth rise significantly. 
     To address the above-described problems, one related-art fixing unit includes a regulator (hereinafter “regulator”) that changes how much of the fixing member is heated along its width (hereinafter also referred to as the heat width of the fixing member) depending on the width of the recording sheet being printed. This fixing unit employs an induction heating method using electromagnetic induction. In the induction heating method, a coil generates a magnetic field serving as a heat source. 
     The regulator consists of a heating portion and a regulating portion. Ferrite is used in the heating portion and copper used in the regulating portion of the regulator, thereby producing a difference in heating efficiency and thus regulating the heat width of the heating member. 
     Furthermore, the regulator rotates in association with the recording sheet so as to adjust the heat width of the heating member, thus changing the heat width of the fixing member to match the width of the recording sheet. As a result, a significant rise in temperatures at the end portions of the heating member, the fixing member, and so forth can be prevented. 
     However, there is a drawback to this configuration when printing is performed continuously on recording sheets of various sizes (for example, A4, A3, A5, B4) and different kinds (for example, an envelope, a paper sheet, etc.). 
     For example, when printing is performed alternately on an envelope and a different kind of a printable medium from the envelope such as a sheet of paper, or when printing is performed alternately in vertical and horizontal directions on a plurality of recording sheets of the same size fed from a plurality of sheet feeders (generally known as “rotary sorting”), printing is continuously performed on the recording sheets of different widths. 
     In order to continuously print an image on the recording sheets of different sizes having different widths, in other words, when the image forming apparatus that can accommodate an A3-size recording sheet continuously performs printing on the A3 recording sheet, then the A5 recording sheet, and then the A4 recording sheet sequentially, in that order, without changing the heat width of the fixing unit for each recording sheet for the sake of maintaining a certain print speed throughout the printing, either a significant rise in temperatures or insufficient heat at the end portions of the heating member, the fixing member, and so forth, occurs. 
     In this regard, the heat width needs to be changed for every sheet, necessitating extra time for changing the heat width. Consequently, after the first sheet is fed, a certain amount of time needs to be provided before the second sheet is fed, thereby reducing printing speed for continuous printing. 
     In view of the above, various approaches have been proposed in an attempt to solve the problem described above. 
     For example, in one conventional heating member, in order to prevent temperature rises in areas where the recording sheet does not contact the fixing member, a driving speed is switched from a normal speed to a low-speed mode only when a relatively small-size recording sheet is specifically designated for printing by the image forming apparatus, or, if the apparatus is part of a network, by a host computer (server) that controls printing. By contrast, when there is no designation of the small-size recording sheet, the driving speed is set at a normal speed. 
     Thus, even when a detector detects an actual recording sheet being a small size after the device starts to be driven at the normal speed, the driving speed is still not switched to the low-speed mode for the small sized paper, and throughput control is performed corresponding to the small sized recording sheet at the normal speed. 
     With this configuration, no detector for detecting the size of the recording sheet is necessitated at an opening from which the recording sheet is fed, thereby providing an inexpensive configuration that still allows the low-speed mode to be set easily. Furthermore, under any conditions, the fixing device does not fail due to temperature rises where the recording sheet is not present. 
     However, there is also a drawback to the foregoing approach in that when the small-sized recording sheet is designated by the user, switching the driving speed to the low-speed mode, the printing speed for continuous printing is forced to slow down. 
     Furthermore, when switching the driving speed from the normal-speed mode to the low-speed mode, extra time is required for switching the mode, thereby also slowing down the printing speed for continuous printing. 
     In another related-art approach to prevent reduction of the printing speed for the first page to be printed, both a relatively large heater as well as a small heater are provided in the image forming apparatus and heating is switched between the two according to paper size. In the image forming apparatus of this approach, when fixing the first sheet after printing is initiated, a CPU for controlling operation of the image forming apparatus always supplies power to the large heater regardless of the paper size. 
     After the small-sized recording sheet is fixed, the large heater is turned on. Furthermore, after the small-sized recording sheet is fixed, the CPU not only turns on the large heater, but also measures the temperature of the heater and stops transport of the recording sheet when the temperature of the heater does not reach a predetermined temperature. Fixing is resumed when the temperature of the heater reaches the predetermined temperature. 
     A drawback to the foregoing configuration is that, because the large heater is turned on before the first printing and after the small-sized recording sheet is printed, a significant rise in temperatures occurs at the end portions of the fixing member, the auxiliary fixing member, the heat source, and so forth. Moreover, when printing is alternately performed between the large-sized paper and the small-sized paper, problems such as uneven fixation and deterioration of the fixing member occur. 
     Furthermore, according to the foregoing technology, two heaters, both the large-sized heater and the small-sized heater, are necessitated, thereby increasing the cost of the image forming apparatus as a whole. In addition, generally, in order to maintain imaging quality, the fixing unit is periodically replaced, causing the cost per paper to increase despite the effort of improving the printing speed for continuous printing. 
     In order to overcome such problems, when printing out a plurality of the recording sheets, the recording sheets are divided into different groups by width, and the group to which the preceding recording sheet belongs is compared with the group to which the subsequent recording sheet belongs. 
     Only when the amount of heat generated in the vicinity of the end portions of the fixing member, the auxiliary fixing member, the heat source, and so forth is not sufficient, a relatively long interval is provided before subsequent transport of transfer paper is started. Other than that, continuous printing is performed at predetermined intervals. Accordingly, a certain printing speed can be maintained when continuously printing, while a significant rise in temperatures and/or insufficient heating can be prevented. 
     However, there is also a drawback to this approach. Since the recording sheet in the preceding group and the recording sheet in the subsequent group are compared with each other, and the relatively long interval is provided before the subsequent transport of the recording sheet is started, the timing with which the recording sheet is transported varies in each case, thus decreasing overall printing speed. 
     SUMMARY OF THE INVENTION 
     In view of the foregoing, in one illustrative embodiment of the present invention, an image forming apparatus for forming an image includes an image bearing member, a developing device, a transfer device, a fixing device, a sheet feeder, a first control table, and a second control table. The image bearing member is configured to bear an electrostatic latent image on a surface thereof. The developing device is configured to develop the electrostatic latent image formed on the image bearing member using toner to form a toner image. The transfer device is configured to transfer the toner image onto a recording sheet. The fixing device is configured to fix the toner image. The sheet feeder is configured to feed the recording sheet. The first control table includes information on widths of the recording sheet categorized into a plurality of groups of different widths thereof in a direction perpendicular to a sheet transport direction. The second control table includes information on timing of the recording sheet transported to the fixing device based on the group to which a preceding recording sheet belongs and the group to which a subsequent recording sheet belongs. When a certain width of the recording sheet belongs to the plurality of the groups in the first control table, the group to which the preceding recording sheet belongs is the group to which the subsequent recording sheet belongs. 
     Additional features and advantages of the present invention will be more fully apparent from the following detailed description of illustrative embodiments, the accompanying drawings and the associated claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete appreciation of the disclosure and many of the attendant advantages thereof will be readily obtained as the same becomes better understood by reference to the following detailed description of illustrative embodiments when considered in connection with the accompanying drawings, wherein: 
         FIG. 1  is a schematic diagram illustrating a color printer as an example of an image forming apparatus according to an illustrative embodiment of the present invention; 
         FIG. 2  is a schematic diagram illustrating a heat-width adjuster of a fixing unit employed in the image forming apparatus of  FIG. 1  according to the illustrative embodiment of the present invention; 
         FIG. 3  is a side view illustrating the heat-width adjuster of  FIG. 2  according to the illustrative embodiment of the present invention; 
         FIG. 4  is a flowchart showing an example of a sheet transport procedure according to the illustrative embodiment of the present invention; 
         FIG. 5  is a block diagram partially illustrating a controller serving as a sheet information processor in the image forming apparatus according to the illustrative embodiment of the present invention; 
         FIG. 6  is an example of a second control table according to the illustrative embodiment of the present embodiment; 
         FIG. 7  is an example of a table for showing classification of a width of a recording sheet according to another illustrative embodiment of the present embodiment; 
         FIG. 8  is an example of a second control table according to another illustrative embodiment of the present invention; 
         FIG. 9  is a flowchart showing an example of a sheet transport procedure according to another illustrative embodiment of the present invention; 
         FIG. 10  is an example of a second control table according to another illustrative embodiment of the present invention; and 
         FIG. 11  is an example of a first control table according to another illustrative embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF ILLUSTRATIVE EMBODIMENTS 
     In describing illustrative embodiments illustrated in the drawings, specific terminology is employed for the sake of clarity. However, the disclosure of this patent specification is not intended to be limited to the specific terminology so selected, and it is to be understood that each specific element includes all technical equivalents that operate in a similar manner and achieve a similar result. 
     Illustrative embodiments of the present invention are now described below with reference to the accompanying drawings. 
     In a later-described comparative example, illustrative embodiment, and alternative example, for the sake of simplicity of drawings and descriptions, the same reference numerals will be given to constituent elements such as parts and materials having the same functions, and redundant descriptions thereof omitted. 
     Typically, but not necessarily, paper is the medium from which is made a sheet on which an image is to be formed. It should be noted, however, that other printable media are available in sheet form, and accordingly their use here is included. Thus, solely for simplicity, although this Detailed Description section refers to paper, sheets thereof, paper feeder, etc., it should be understood that the sheets, etc., are not limited only to paper, but includes other printable media as well. 
     Referring now to the drawings, wherein like reference numerals designate identical or corresponding parts throughout the several views, and initially to  FIG. 1 , one example of an image forming apparatus according to an illustrative embodiment of the present invention is described. 
     Referring now to  FIG. 1 , there is provided a schematic diagram illustrating a color printer of a tandem direct-transfer type as one example of the image forming apparatus  10 . 
     As illustrated in  FIG. 1 , the image forming apparatus  10  includes three sheet feed trays for supplying recording media P including a manual sheet feed tray  20 , sheet cassettes  21  and  22 , a sheet feeder  23 , a pair of registration rollers  8 , a separation member  24 , a pair of transport rollers  25 , a transfer belt  52 , transfer rollers  51 M,  51 C,  51 Y, and  51 Bk, and so forth. 
     The image forming apparatus  10  includes image forming stations for colors of black (Bk), yellow (Y), cyan (C), and magenta (M). The image forming stations for black, yellow, cyan, and magenta all have the same configuration, differing only in the color of toner employed. Thus, the description is thereafter provided of the image forming station for the color black as a representative example of the image forming stations. 
     It is to be noted that reference characters Bk, Y, C, and M denote the colors black, yellow, cyan, and magenta, respectively. 
     The image forming station for black includes the photoreceptor drum  1 Bk, a charging roller  2 Bk for charging the photoreceptor drum  1 Bk, a developing device  4 Bk, and a cleaning device  6 Bk for removing residual toner from the surface of the photoreceptor drum  1 Bk.  3 Bk denotes an optical axis of a laser beam illuminating the photoreceptor drum  1 Bk. An optical writing unit  3  projects the laser beam against the photoreceptor drum  1 Bk. The image forming station for yellow includes the photoreceptor drum  1 Y, a charging roller  2 Y for charging the photoreceptor drum  1 Y, a developing device  4 Y, and a cleaning device  6 Y for removing residual toner from the surface of the photoreceptor drum  1 Y. The image forming station for cyan includes the photoreceptor drum  1 C, a charging roller  2 C for charging the photoreceptor drum  1 C, a developing device  4 C, and a cleaning device  6 C for removing residual toner from the surface of the photoreceptor drum  1 C. The image forming station for magenta includes the photoreceptor drum  1 M, a charging roller  2 M for charging the photoreceptor drum  1 M, a developing device  4 M, and a cleaning device  6 M for removing residual toner from the surface of the photoreceptor drum  1 M.  3 Y,  3 C and  3 M denote optical axes of laser beams illuminating the photoreceptor drums  1 Y,  1 C and  1 M, respectively. 
     When feeding manually, recording media sheets P, not illustrated, serving as printable media, are fed from the manual sheet feed tray  20  one sheet at a time. A top sheet of the recording sheet P is separated from the rest of the sheets one sheet at a time by the sheet feeder  23  and transported to the pair of the registration rollers  8 . 
     The recording sheet P being fed is stopped at the pair of the registration rollers  8  and aligned so that skew is corrected. Subsequently, the recording sheet P is transported to the transfer belt  52  in appropriate timing such that the recording sheet P is aligned with a toner image formed on the photoreceptor drum  1 M serving as an image bearing member provided substantially uppermost stream. 
     A suction roller, not illustrated, contacts the transfer belt  52  forming a nip therebetween. When the recording sheet P passes the nip between the transfer belt  52  and the suction roller, bias is applied to the suction roller so that the recording sheet P is electrostatically suctioned to the transfer belt  52  and transported at a predetermined process linear velocity. 
     The recording sheet P suctioned to the transfer belt  52  is applied with a transfer bias, that is, a polarity opposite the polarity of toner, by transfer rollers  51 M,  51 C,  51 Y, and  51 Bk. 
     The transfer rollers  51 M,  51 C,  51 Y, and  51 Bk are disposed corresponding to the photoreceptor drums  1 M,  1 C,  1 Y, and  1 Bk through the transfer belt  52 , and serve as transfer bias application members. Accordingly, toner images formed on the photoreceptor drums  1 M,  1 C,  1 Y, and  1 Bk in magenta (M), cyan (C), yellow (Y), and black (Bk) are sequentially transferred onto the recording sheet P. 
     After the toner images are transferred onto the recording sheet P, the recording sheet P separates from the transfer belt  52  at a drive roller  26  due to its curvature and is transported to a fixing unit  30 . 
     In the fixing unit  30 , a fixing belt  63  serving as a fixing member, and a pressure roller  65  provided opposite the fixing belt  63  and serving as a pressure member sandwich the recording sheet P therebetween. It is to be noted that the place where the fixing belt  63  and the pressure roller  65  meet and press against each other is a so-called fixing nip portion. In the fixing nip portion, heat and pressure are applied to the toner image so that the toner image is fixed onto the recording sheet P. 
     In a case of a single-sided printing, after the toner image is fixed on one side of the recording sheet P, the recording sheet P is discharged onto a sheet discharge tray  33  provided substantially at an upper portion of the image forming apparatus  10 . 
     In a case of duplex printing, after the image is printed on one side of the recording sheet, the recording sheet passes through the fixing unit  30  and is transported to a sheet reverse unit  40  disposed outside the image forming apparatus  10 . In the sheet reverse unit  40 , the sheet transport direction is switched so that an image can be printed on the other side of the recording sheet. The recording sheet is transported to the transfer belt  52  by a duplex transport unit  35  and transport rollers  36  and  25 . 
     The image forming apparatus  10  also includes tension rollers  27  and  28 , and a support roller  29 . The transfer belt  34  is wound around and stretched by the drive roller  26 , the tension rollers  27  and  28 , and the support roller  29 . 
     Referring now to  FIG. 2 , there is provided a schematic diagram illustrating a heat-width changing mechanism including a heat-width adjuster  73  using an electromagnetic induction heating method according to the illustrative embodiment of the present invention.  FIG. 3  is a side view of the heat-width adjuster  73  of  FIG. 2 . 
     The fixing unit  30  of  FIG. 1  includes the heat-width changing mechanism including the heat-width adjuster  73  illustrated in  FIGS. 2 and 3  to change the heat width of a heating member according to the width of the recording sheet P. 
     According to the illustrative embodiment, a magnetic field is generated by a coil  72  serving as a heat source, and the heat width of a heating member  71  is regulated using the heat-width adjuster  73 .  FIG. 2  schematically illustrates the heat-width adjuster  73  according to the illustrative embodiment. 
     As illustrated in  FIG. 3 , the heat-width adjuster  73  includes a heating portion  73   c  made of a material such as ferrite and a regulating member  73   a  having a step shape symmetrical to a regulating member  73   b , and the regulating member  73   b . The regulating members  73   a  and  73   b  are made of a material such as copper. With this configuration, heating efficiencies thereof are different, thereby regulating the heat width of the heating member  71 . 
     When the heat-width adjuster  73  rotates with the recording sheet P, the heat width of the heating member  71  is regulated, thereby enabling the heat width of the fixing member (fixing belt)  63  to be properly adjusted according to the recording sheet P. 
     As described above, the coil  72  generates a magnetic field, and the heat-width adjuster  73  including the heating portion  73   c  made of ferrite or the like and the regulating members  73   a  and  73   b  symmetrical to each other regulate the heat width of the heating member  71 . 
     Transport timing of the recording sheet is determined by receiving width information on the recording sheet from a controller  80  serving as a sheet information processor, a detailed description of which is provided later, and based on a first control table  85  and a second control table  82 . 
     With this configuration, there is no need to provide a detector dedicated to detecting a recording sheet P between either the sheet feeder  23  or the separation member  24  and the pair of registration rollers  8 . Accordingly, the size of the image forming apparatus as a whole as well as its cost can be reduced. 
     Referring now to  FIG. 4 , there is provided a flowchart showing an example procedure of the illustrative embodiment of the present invention.  FIG. 5  is a block diagram illustrating the controller  80  serving as the sheet information processor provided to the image forming apparatus  10 .  FIG. 6  is an example of a timing table serving as the second timing table  82 . 
     Before explaining the flowchart in detail, with reference to  FIG. 5 , a description is provided of the controller  80  serving as an information processor according to the illustrative embodiment. The controller  80  includes a CPU  81 , the first control table  85 , the second control table  82  (timing table), a G1 memory  83 , and a G2 memory  84 . 
     The CPU  81  of the controller  80  processes information on the recording sheet P, separation control, and determination of the sheet transport timing based on the information from the second control table  82 . 
     The first control table  85  includes initial reference sheet information. According to the illustrative embodiment, the first control table  85  includes information on different standard-sized sheets, based on which the width of the recording sheet is initially categorized. 
     The G1 memory  83  stores group information including the width information of the first, or the preceding, recording sheet P. The G2 memory  84  stores group information including the width information of the second, or the subsequent, recording sheet P. 
     When the controller  80  receives print instruction, print data such as character image data, image data, and so forth from a personal computer (PC), the information on the first recording sheet P is referred to the first control table  85  to find an appropriate width group for the first recording sheet. The width group to which the first recording sheet belongs is stored in the G1 memory  83 . 
     The information on the subsequent recording sheet P is referred to the first control table  85 , and the width group to which the recording sheet P belongs is stored in the G2 memory  84 . 
     The group information identified by the first control table  85 , stored in the G1 memory  83 , is compared with the group information identified by the first control table  85 , stored in the G2 memory  84 , using the second control table  82  serving as a timing table. Accordingly, the CPU  81 , which controls sheet transport, fixation, and so forth, determines the sheet transport timing for the subsequent recording sheet P. 
     With reference to  FIG. 1  and  FIGS. 4 through 6 , a description is provided of an example procedure of determination of the sheet transport timing according to the illustrative embodiment. 
     In  FIG. 4 , at S 1 , information on the first recording sheet P is obtained from the controller  80 , and the width of the first sheet is categorized into Group A based on the first control table  85 . 
     According to the illustrative embodiment, the recording sheet P is initially categorized by generally known standard sizes in Group A. Furthermore, the initial group in Group A is divided further into a higher-level group, Group B. In Group B, it is determined whether or not the width of the recording sheet is 200 mm or more. 
     According to the illustrative embodiment, when the recording sheet P is a standard-sized sheet and the width thereof is 200 mm or more, the recording sheet P is categorized as G1=2 in Group B. 
     By contrast, when the recording sheet P is a standard-sized sheet and the width thereof is less than 200 mm, the recording sheet P is categorized as G1=1 in Group B. 
     Then, the sheet information is stored in an internal memory, that is, the G1 memory  83 . 
     In other words, when the preceding recording sheet P is a standard-sized sheet and the width thereof is 200 mm or more, G1=2. The information is stored in the G1 memory  83 . 
     By contrast, when the preceding recording sheet P is a standard-sized sheet and the width thereof is less than 200 mm, G1=1. The information is stored in the G1 memory  83 . 
     After the processing described above, at S 2 , transport of the preceding or the first recording sheet P is initiated. 
     Alternatively, sheet transport can be initiated without categorizing the recording sheet. In this case, an interval between transport of the preceding recording sheet and transport of the subsequent recording sheet is set based on the control table. After a predetermined time, transport of the subsequent sheet is initiated. 
     Furthermore, alternatively, transport timing of the recording sheet may be determined simply by categorizing the recording sheet into Group A, that is, categorizing the recording sheet by generally known standard sizes. 
     Next, a description is provided of a case in which the recording sheet P is fed from the manual sheet tray  20 . Referring back to  FIG. 1 , the top sheet of the recording sheets on the manual sheet feed tray  20  is separated one sheet at a time by the sheet feeder  23  and is sent to the pair of registration rollers  8  through the pair of the transport rollers  25 . 
     By contrast, when the recording sheet P is fed either from the sheet cassette  21  or the sheet cassette  22 , the separation member  24  picks up the top sheet from the sheet cassette  21  or the sheet cassette  22 , and sends the recording sheet P to the pair of registration rollers  8  through the pair of transport rollers  25 . 
     Subsequently, at S 3 , it is determined whether or not the recording sheet being transported is the last sheet. If it is the last sheet, the procedure is finished. By contrast, when it is not the last sheet, the group information on the width of the recording sheet, identified with the first control table  85 , is stored in the G1 memory  83  of the controller  80  at S 4 . 
     Subsequently, at S 5 , the information on the second or the subsequent sheet of the recording sheet P is obtained from the controller  80 , and the width of the second recording sheet P is categorized by referring to the first control table  85 . 
     Similar to the first sheet, the second or the subsequent recording sheet P is categorized by generally known standard size so as to determine the initial group in Group A. Furthermore, in Group B, it is determined whether or not the width of the recording sheet is 200 mm or more. 
     When the width of the second or the subsequent sheet is 200 mm or more, the second sheet is categorized as G2=2 in Group B. At S 6 , the information on the second or the subsequent sheet P is stored in the internal memory, that is, the G2 memory  84 . 
     By contrast, when the width of the second or the subsequent sheet is less than 200 mm, the second sheet is categorized as G2=1 in Group B. At S 6 , the information on the second sheet is stored in the internal memory, that is, the G2 memory  84 . 
     Subsequently, at S 7 , in accordance with the second control table  82  serving as a timing table, the values of G1 and G2 obtained from the first control table  85  are compared with each other, and transport timing for the second or the subsequent sheet is determined. 
       FIG. 6  illustrates one example of the second control table  82 . 
     According to the illustrative embodiment, when G1≧G2, the transport timing t is 0.2 seconds (t=0.2 sec.). When G1&lt;G2, the transport timing t 1  is 4 seconds (t 1 =4 seconds). 
     The information on the length of the first recording sheet is obtained from the controller  80  or based on duration of signals from a sheet detector, not illustrated, that detects presence of the recording sheet P. The sheet detector can be provided to the sheet feeder  23  or the separation member  24 . 
     Then, at S 8 , a stand-by time t 2 , that is, an interval between the start of transport of the first or the preceding sheet and the start of transport of the second or the subsequent sheet, is calculated by the CPU  81 . 
     For example, when the transport timing t 1  is 0.2 seconds (t 1 =0.2 sec.) and the first sheet of the recording sheet P is a standard A4 sheet fed in landscape orientation, the length of the A4 sheet in the direction of transport is approximately 210 mm. 
     Landscape orientation herein refers to a horizontal print orientation in which the long side of the sheet is at the top. Thus, when the transport speed of the image forming apparatus in the transport direction is approximately 150 mm/sec, t 2  is 1.6 seconds. (t 2 =0.2+210/150=1.6 seconds) 
     Subsequently, after transport of the first or the preceding sheet is initiated, the stand-by time t 2  is provided at S 9 . Then, transport of the second or the subsequent sheet is initiated. 
     Accordingly, when the first and the second recording sheets are categorized into groups based on the sheet widths thereof and then the group G1 of the first recording sheet P is compared with the group G2 of the second recording sheet, the transport timing t 1  is determined. 
     Based on the result, the stand-by time t 2  is determined. Only when there is a significant difference between the width of the first or the preceding recording sheet and the width of the second or the subsequent recording sheet, that is, G1≠G2, the stand-by timing t 2  is changed. 
     By contrast, when the difference between the width of the first or the preceding recording sheet and the width of the second or the subsequent recording sheet is insignificant, that is, G1=G2, t 2  can be set to a relatively short time which is substantially the same interval as in the case of continuous printing of the same size recording media sheets P. According to the present embodiment, t 2  is set to approximately 0.2 seconds, for example. 
     With this configuration, the stand-by time t 2  is less likely to be longer than the interval for continuous printing. Furthermore, while the present invention is employed and printing in landscape orientation and portrait orientation is properly carried out to correspond to the sheet width so as to reduce fluctuation of the width of the recording sheets, the stand-by time can be reduced. 
     When the information of the recording sheet including the width information is obtained from the controller  80  and the transport timing is determined based on the first control table  85  and the second control table  82 , the sheet detector for detecting the recording sheet P does not have to be provided at the sheet feeder  23  or the separation member  24 , or between the pair of the registration rollers  8 . Thus, reduction in both size and cost of the image forming apparatus can be achieved. 
     With reference to  FIGS. 1 through 3 , a description is provided of another illustrative embodiment of the present invention. 
     The image forming apparatus of the present embodiment includes the fixing unit  30  equipped with the coil  72  serving as the heat source, the heating member  71 , the heat-width adjuster  73  that regulates the heat width of the heating member  71 , and an adjuster controller  41  that controls the heat-width adjuster  73 . 
     Upon continuous printing, during the time from which the preceding recording sheet P passes the fixing unit  30  to which the subsequent recording sheet arrives at the fixing unit  30 , the adjuster controller  41  controls the heat-width adjuster  73  such that the heat width of the heating member  71  becomes an appropriate width for the subsequent recording sheet. 
     In other words, in the image forming apparatus  10  of the foregoing embodiment, immediately after or substantially at the same time as the first or the preceding recording sheet passes the fixing unit  30 , the heat-width adjuster  73  of the fixing unit  30  is rotated so as to regulate the heat width of the heating member  71  to associate with the width of the second recording sheet. 
     At this time, when the stand-by time t 2  of the second control table  82  is determined to correspond to the time of start and finish of rotation of the heat-width adjuster  73 , the heat width corresponding to the width of the recording sheet is attained. 
     With regard to the stand-by time t 2 , in a case in which the difference between the width of the first sheet and the width of the second sheet of the recording sheet is insignificant, that is, for example, when the first sheet belongs to  257  of the initial width group A and the second sheet belongs to  210  of the initial width group A, the stand-by time t 2  is set at minimum in order to prevent the printing speed from slowing down during continuous printing. For example, according to the present embodiment, t 1  is 0.2 seconds (t 1 =0.2 sec.). 
     By contrast, in a case in which there is a significant difference between the width of the first sheet and the width of the second sheet of the recording sheet, problems such as significant rise in temperatures or insufficient heat in the vicinity of the end portions of the heat source  72 , the heating member (the auxiliary fixing member)  71 , the fixing belt  63 , and so forth occurs. For this reason, it is necessary to properly adjust the heat-width adjuster  73  to correspond to the width of the recording sheet before the subsequent recording sheet arrives at the fixing device  30 . 
     When the difference between the width of the first sheet and the width of the second sheet of the recording sheet is insignificant, even if adjustment of the heat-width adjuster  73  is not completed at the time of arrival of the subsequent recording sheet at the fixing unit  30 , the heating area that exceeds the width of the recording sheet is relatively small. Thus, the significant rise in temperatures or insufficient heat in the vicinity of the end portions of the heat source  72 , the heating member (the auxiliary fixing member)  71 , the fixing belt  63 , and so forth does not cause a serious problem. 
     In this regard, when grouping of the first control table  85  is determined in accordance with output of the heat source  72 , sheet transport speed, a heat capacity of the fixing belt  63  and the auxiliary fixing member, the stand-by time t 2  can be prevented from exceeding the interval for continuous printing of the same-size recording media sheets without significant rise in temperatures or insufficient heat in the vicinity of the end portions of the heat source  72 , the heating member (the auxiliary fixing member)  71 , the fixing belt  63 , and so forth. 
     A description is now provided of another illustrative embodiment (Embodiment 3). 
     Referring now to  FIG. 7 , there is provided a table for showing classification of the width of the recording sheet.  FIG. 8  is a table serving as the second control table  82 . 
     According to Embodiment 3, information on the width and the type of recording sheet are obtained from the controller  80 , and the table as shown in  FIG. 8  is employed as the second control table  82 . 
     The timing of transport of the recording sheet is determined based on the first control table  85  and the second control table  82 , using the information on the width of the recording sheet in the direction perpendicular to the sheet transport direction and the type of recording sheet obtained from the controller  80  illustrated in  FIG. 5 . 
     According to the present embodiment, in addition to the width information of the recording sheet, the information on the type of recording sheet is obtained from the controller  80 , and the transport timing of the recording sheet is determined based on the first control table  85  and the second control table  82 . 
     When the information on the type of recording sheet as well as the information on the width thereof is obtained from the controller  80  and the sheet transport timing t 1  and the stand-by time t 2  are determined based on the second control table  85 , the time required for changing the setting of the image forming apparatus can be secured even if the type of recording sheets varies. When the transport timing for the recording sheet is properly set, the printing speed can be prevented from slowing down during the continuous printing. 
     A description is now provided of another illustrative embodiment (Embodiment 4). 
     With reference to  FIGS. 7 and 8 , a description is provided of continuous printing of a normal sheet and then a relatively thick sheet, according to the present embodiment. 
     When continuously printing out the first or the preceding recording sheet of a normal A4 sheet fed in landscape orientation and the second or the subsequent recording sheet of a relatively thick A5 sheet fed in portrait orientation, the first sheet is categorized as G1=2 for the width of 200 mm or greater, and the second sheet is categorized as G2=1 for the width less than 200 mm. Thus, G1 is greater than G2 (G1&gt;G2), and the sheet transport timing t 1  is 6 seconds (t 1 =6 seconds). 
     In accordance with this sheet transport timing, the stand-by time t 2 , that is, the time from the start of transport of the first sheet to the start of transport of the second sheet, is calculated. 
     According to the present embodiment, the image forming apparatus includes the fixing unit  30  equipped with the heat source  72 , the heating member  71 , the heat-width adjuster  73  that regulates the heat width of the heating member  71 , and the adjuster controller  41  that controls the heat-width adjuster  73  and the heat source  72 . 
     Upon continuous operation, during the time from when the preceding recording sheet passes the fixing unit  30  to when the subsequent recording sheet arrives at the fixing unit  30 , the adjuster controller  41  controls the heat-width adjuster  73  and the heat source  72  such that the heat-width adjuster  73  is regulated to the proper heat width for the subsequent recording sheet, and the heat source  72  is regulated according to the type of recording sheet. 
     Immediately after or substantially at the same time as the first recording sheet passes the fixing unit  30 , the adjuster controller  41  causes the heat-width adjuster  73  of the fixing unit  30  to rotate so that the heat width is regulated to the proper heat width for the second recording sheet. Furthermore, the adjuster controller  41  controls the heat source  72  to a proper temperature for the type of second recording sheet. 
     According to the present embodiment, because more heat is needed for the thick sheet than for the normal sheet, in a case in which the first sheet is a normal sheet and the second sheet is a thick sheet, the sheet transport timing t 1  is set relatively long so that the fixing belt  63  and the heating member or the auxiliary fixing member  71  can accumulate heat. 
     As described above, when the type of recording sheet varies, the amount of heat necessary for fixation differs as well. Thus, it is necessary to change the amount of heat accumulated by the fixing belt  63  and the heating member (the auxiliary fixing member)  71  by changing the output and/or duty cycle of the heat source  72 . 
     In view of this, when the time required for changing the output and/or duty cycle of the heat source  72  is incorporated in the sheet transport timing t 1  and the stand-by timing t 2  of the second control table  85  of  FIG. 5  in advance, significant rises in temperature or insufficient heat in the vicinity of the end portions of the heat source  72 , the heating member (the auxiliary fixing member)  71 , the fixing member  63 , and so forth can be prevented, and further, deterioration of performance during continuous printing can be prevented. 
     With reference to  FIG. 9 , a description is provided of another illustrative embodiment (Embodiment 5).  FIG. 9  is a flowchart showing an example procedure of the present embodiment. 
     At S 1 , information on the first or the preceding sheet of the recording sheet is obtained from the controller  80 , and the width information of the first sheet is referred to the first control table  85  so as to categorize the width of first sheet in Group A. 
     After the procedure shown in the flowchart of  FIG. 4  is performed, at S 12 , transport of the first sheet is initiated. In a case of manually feeding the first sheet, the top sheet of the recording sheets on the manual sheet feed tray  20  is transported to the pair of the registration rollers  8  by the sheet feeder  23 . 
     In a case of feeding the first sheet from the sheet cassette  21  or the sheet cassette  22 , the separation member  24  picks up the top sheet and sends it to the registration rollers  8  through the pair of the transport rollers  25 . 
     Subsequently, at S 13 , whether or not the sheet being transported is the last sheet is determined. When it is the last sheet, the procedure is finished. By contrast, when it is not the last sheet, at S 14 , based on the first control table  85 , the group information of the width of the first sheet is stored in the G1 memory  83  of the controller  80 . 
     Subsequently, at S 15 , information on the second sheet is obtained from the controller  80  and referred to the first control table  85  so as to categorize the width of the second sheet in Group A. 
     Similar to the first sheet, the second sheet is categorized by generally known standard sizes, and the initial group, to which the second sheet belongs, is determined in Group A. Furthermore, when the width of the second sheet in Group A is 200 mm or greater, G2 is 2 (G2=2) in Group B. By contrast, when the second sheet is less than 200 mm, G2 is 1 (G2=1) in Group B. Subsequently, at S 16 , the group information of the width of the second sheet is stored in the G2 memory  84 . 
     Subsequently, at S 17 , using the second control table  82  serving as the timing table, the results of G1 and G2 obtained from the first control table  85  are compared, and whether or not G1 is equal to or greater than G2 is determined (G1≧G2). 
     When G1≧G2, it is determined at S 18  that continuous transport is possible, and the procedure returns to S 12  and execute the procedure assuming t=0.2 seconds, for example. 
     Referring now to  FIG. 10 , there is provided an example of a timing table serving as the second control table  82 . 
     When G1 is not equal to or greater than G2 at S 17 , that is, G1 is less than G2 (G1&lt;G2), it is determined that continuous sheet transport cannot be performed at S 19 , and transport of the second sheet is initiated after the first sheet passes the fixing unit  30 . 
     As described above, when G1 is equal to or greater than G2 (G1≧G2), it is determined that continuous sheet transport can be carried out. Since the width of the first sheet is relatively wide and the width of the subsequent sheet is less than that of the first sheet, and further, the difference between the widths of the first and the second sheets is relatively small, the amount of heat in the area outside the end portions of the sheet becomes temporarily excessive when the portion of the subsequent sheet in the vicinity of the leading end thereof is being fixed. 
     However, after the heat-width adjuster  73  illustrated in  FIG. 2  is regulated such that the heat width corresponds to the width of the recording sheet, the area outside the recording sheet is prevented from being heated, thereby reducing temperatures thereat. 
     Accordingly, significant rises in temperature and/or insufficient heat can be prevented without extending the sheet transport timing t 1  and the stand-by time t 2  more than necessary. 
     In this case, the detector that detects presence of the recording sheet may be provided immediately after the fixing unit  30 . Alternatively, as shown in the table of  FIG. 10 , the transport timing for the second sheet can be set in advance so that the detector dedicated for detecting presence of the recording sheet is not necessary. 
     With reference to  FIG. 11 , a description is now provided of another embodiment (Embodiment 6). Referring now to  FIG. 11 , there is provided a table for showing an example of the control table serving as the first control table  85 . 
     According to the present embodiment, when the recording sheet is categorized by referring to the first control table  85 , an arbitrary width of the recording sheet in the initial Group A in the first control table  85  belongs to a plurality of groups in Group B. For example, a group  210  of the group A belongs to both G=1 and G=2 in Group B When the first sheet is categorized into a group  257  in Group A, G1 is 2 (G1=2). When the second sheet is categorized into the group  210  in Group A, the group  210  belongs to both G=1 and G=2, and the same group as that of the preceding recording sheet is prioritized and selected. As a result, G2 is G1 (G2=G1), thus achieving t 1 =0.2 seconds. 
     As can be understood, when the frequently-used width group in Group A is shared by Group B, risk of degradation of continuous printing can be effectively reduced. 
     For example, in a case in which a plurality of sheets of different sizes are continuously printed out in the following order, for example, the first sheet is an A5 sheet and fed in landscape orientation, the second sheet is an A6 sheet and fed in portrait orientation, the third sheet is an A5 sheet and fed in landscape orientation, and the fourth sheet is an A4 sheet and fed in landscape orientation, and when the table as shown in  FIG. 7  is used as the first control table  85  while the table as shown in  FIG. 10  is used as the second control table  82 , the sheet transport timing for the second and the third sheets is t 1 =4 seconds, which is relatively long. 
     However, when the table as shown in  FIG. 11  is employed for the first control table  85 , the sheet transport timing for all the sheets becomes 0.2 seconds without undesirably extending the transport timing t 1  and the stand-by time t 2 . Furthermore, significant rise in the temperature or insufficient heat at the end portions of the heating member, the fixing member, and so forth can be prevented. 
     According to the illustrative embodiment, the present invention is employed in the image forming apparatus. The image forming apparatus includes, but is not limited to, an electrophotographic image forming apparatus, a copier, a printer, a facsimile machine, and a digital multi-functional system. 
     Furthermore, it is to be understood that elements and/or features of different illustrative embodiments may be combined with each other and/or substituted for each other within the scope of this disclosure and appended claims. In addition, the number of constituent elements, locations, shapes and so forth of the constituent elements are not limited to any of the structure for performing the methodology illustrated in the drawings. 
     Still further, any one of the above-described and other exemplary features of the present invention may be embodied in the form of an apparatus, method, or system. 
     For example, any of the aforementioned methods may be embodied in the form of a system or device, including, but not limited to, any of the structure for performing the methodology illustrated in the drawings. 
     Example embodiments being thus described, it will be obvious that the same may be varied in many ways. Such exemplary variations are not to be regarded as a departure from the scope of the present invention, and all such modifications as would be obvious to one skilled in the art are intended to be included within the scope of the following claims.