Patent Publication Number: US-10317831-B2

Title: Image forming system, image forming method, imaging forming apparatus, job management method, and computer-readable non-transitory recording medium strong job management program

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     The present invention claims priority under 35 U.S.C. § 119 to Japanese Patent Application No. 2017-47082, filed on Mar. 13, 2017, which is incorporated herein by reference in its entirety. 
     BACKGROUND 
     1. Technological Field 
     The present invention relates to an image forming system, an image forming method, an image forming apparatus, a job management method, and a computer-readable non-transitory recording medium storing a job management program. 
     2. Description of the Related Art 
     An image forming apparatus (printer, copying machine, facsimile, or the like) using electrophotographic technology performs a fixing process of passing (heating under pressure) toner images transferred onto a sheet by an image forming section through a fixing nip of a fixing section and discharges the sheet subjected to the fixing process out of the apparatus. 
     Recently, there has been a demand to print an image on a partially thickened sheet such as a chip-embedded sheet using an image forming apparatus such as described above. Compared to a sheet of uniform thickness, the chip-embedded sheet has a very large difference (height difference) between thick part and thin part. Consequently, in printing toner images on a sheet such as a chip-embedded sheet, there is a problem in that if a fixing process is performed using normal fixing pressure (fixing load), durability of fixing members, such as rollers, forming a fixing nip is decreased. 
     A technique for dealing with problems peculiar to image formation on such a special sheet has not yet been proposed. For example, Japanese Patent Application Laid-Open No. 2010-152129 (hereinafter referred to as PTL 1) discloses a technique for changing a gloss level of an image formed on the same side of a same recording sheet as follows: images are formed by dividing the gloss level into plural sublevels according to desired units of gloss level change and the number of fixing processes is controlled to vary among the formed images. However, the technique described in PTL 1 assumes the use of a normal sheet uniform in sheet thickness, and cannot solve the above-mentioned problem in that the durability of fixing members is decreased when printing an image on a partially thickened sheet such as a chip-embedded sheet. 
     In printing on recording material such as chip-embedded sheet, as a means of ensuring the durability of fixing members, it is conceivable to reduce the pressure (fixing load) of the fixing nip against the recording material during the fixing process. On the other hand, when the fixing load is reduced, there is a problem in that although in a thickened region in which the IC chip is placed, fixability is ensured by nip surface pressure increased during passage of a sheet, in a thin region in which no IC chip is placed, fixability cannot be ensured because of low nip surface pressure. 
     SUMMARY 
     An object of the present invention is to provide an image forming system, an image forming method, an image forming apparatus, a job management method, and a computer-readable non-transitory recording medium storing a job management program, where when forming an image on a partially thickened sheet nonuniform in sheet thickness, the image forming system, image forming method, image forming apparatus, job management method, and job management program can ensure good fixability and ensure durability of members forming a fixing nip. 
     To achieve at least one of the aforementioned objects, according to an aspect of the present invention, an image forming system reflecting one aspect of the present invention comprises a first image forming apparatus and a second image forming apparatus, the first and the second image forming apparatuses including respective image formers and fixers, where the image forming system forms an image on a sheet using the first image former, and then forms an image on the sheet using the second image forming apparatus, wherein,
         in a case where an image is formed on a sheet which is nonuniform in sheet thickness and which includes a first region having a small sheet thickness and a second region having a large sheet thickness,   the image in the first region is formed and fixed using the first image forming apparatus and the image in the second region is formed and fixed using the second image forming apparatus, and   the respective fixers of the first and the second image forming apparatuses pass the sheet which is nonuniform in sheet thickness, while applying a second load as a fixing load, lower than a first load applied to a sheet which is uniform in sheet thickness.       

     To achieve at least one of the aforementioned objects, according to an aspect of the present invention, an image forming method reflecting another aspect of the present invention is a method for forming, using an image forming apparatus including an image former and a fixer, an image on a sheet which is nonuniform in sheet thickness and which includes a first region having a small sheet thickness and a second region having a large sheet thickness, the image forming method comprising
         forming and fixing an image in the first region in a first printing process and forming and fixing an image in the second region in a second printing process, wherein   in the first and the second printing processes, the fixer passes the sheet which is nonuniform in sheet thickness, while applying a second load as a fixing load, lower than a first load applied to a sheet which is uniform in sheet thickness.       

     To achieve at least one of the aforementioned objects, according to an aspect of the present invention, an image forming apparatus reflecting another aspect of the present invention comprises:
         an image former that forms a toner image on a sheet;   a fixer that fixes the toner image on the sheet; and   a hardware processor that controls, in a case where the image is formed on a sheet which is nonuniform in sheet thickness and which has a first region having a small sheet thickness and a second region having a large sheet thickness, the image former and the fixer such that a toner image in the first region is formed and fixed in a first printing process and an image in the second region is formed and fixed in a second printing process, wherein   the hardware processor controls the fixer so as to pass the sheet which is nonuniform in sheet thickness and to apply at the same time a second load as a fixing load in the first and the second printing processes, lower than a first load applied to a sheet which is uniform in sheet thickness.       

     To achieve at least one of the aforementioned objects, according to an aspect of the present invention, a job management method reflecting another aspect of the invention is a method for assigning a print job to printing apparatuses each being configured to form an image on a sheet and registered on a network, each of the printing apparatuses being either a tandem machine including first and second image forming apparatuses including respective image formers and fixers, or a single-machine including an image former and a fixer, wherein,
         when a print job of forming an image on a sheet which is nonuniform in sheet thickness and which has a first region having a small sheet thickness and a second region having a large sheet thickness is received,   at least one of the printing apparatuses that executes the print job is specified by taking operating statuses of the registered printing apparatuses into consideration, and the specified printing apparatus is instructed to:   form and fix an image in the first region in a first printing process and form and fix an image in the second region in a second printing process; and   apply a second load as a fixing load in the first and the second printing processes, lower than a first load applied to a sheet which is uniform in sheet thickness.       

     To achieve at least one of the aforementioned objects, according to an aspect of the present invention, a computer-readable non-transitory recording medium reflecting another aspect of the present invention is a medium storing a job management program executed by a server that assigns a print job to printing apparatuses each being configured to form an image on a sheet and registered on a network, each of the printing apparatuses being either a tandem machine including first and second image forming apparatuses including respective image formers and fixers, or a single-machine including an image former and a fixer, wherein the job management program causes the server to carry out a procedure comprising:
         specifying at least one of the printing apparatuses that executes the print job, by taking operating status of the registered printing apparatuses into consideration, upon receiving a print job of forming an image on a same side of a sheet which is nonuniform in sheet thickness and which has a first region having a small sheet thickness and a second region having a large sheet thickness; and   instructing the specified printing apparatus to form and fix an image in the first region in a first image formation process and form and fix an image in the second region in a second image formation process and apply a second load in the first and the second printing processes, lower than a first load applied to a sheet which is uniform in sheet thickness.       

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The advantages and features provided by one or more embodiments of the invention will become more fully understood from the detailed description given hereinbelow and the appended drawings which are given by way of illustration only, and thus are not intended as a definition of the limits of the present invention, and wherein: 
         FIG. 1  shows an overall configuration of an image forming system according to Embodiment 1; 
         FIG. 2  is a functional block diagram of the image forming system of  FIG. 1 ; 
         FIG. 3  is diagram showing configurations of an image forming section and fixing section in the image forming system of  FIG. 1 ; 
         FIG. 4  is a partial sectional view explaining a configuration of a chip-embedded sheet; 
         FIG. 5  is a flowchart explaining a processing flow for forming an image on a chip-embedded sheet in the image forming system of  FIG. 1 ; 
         FIG. 6  shows an overall configuration of an image forming apparatus according to Embodiment 2; 
         FIG. 7  is a functional block diagram of the image forming apparatus of  FIG. 6 ; 
         FIG. 8  is a flowchart explaining a processing flow for forming an image on a chip-embedded sheet in the image forming apparatus of  FIG. 6 ; 
         FIG. 9  is diagram showing an overall configuration of a network system according to Embodiment 3; and 
         FIG. 10  is a flowchart explaining a processing flow for forming an image on a chip-embedded sheet in the network system of  FIG. 9 . 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, one or more embodiments of the present invention will be described with reference to the drawings. However, the scope of the invention is not limited to the disclosed embodiments. 
     Embodiment 1 
       FIG. 1  shows an overall configuration of image forming system  10  according to Embodiment 1. Image forming system  10  is made up of sheet feed unit PFU, first image forming apparatus  100 , second image forming apparatus  200 , post-processing apparatus  300 , and the like, which are connected in sequence. First image forming apparatus  100  includes reversal mechanism R 1  while second image forming apparatus  200  includes reversal mechanism R 2 . Post-processing apparatus  300  includes sheet output tray  310 . The arrow in  FIG. 1  indicates a sheet conveyance path. A system made up of two or more image forming apparatuses connected in series, such as image forming system  10  shown in  FIG. 1 , is generally called a series-tandem image forming system. 
     First, a case in which images are printed on a sheet uniform in sheet thickness (hereinafter also referred to as a normal sheet) will be described. In the case of simplex printing on a normal sheet, image forming system  10  feeds a sheet from sheet feed unit PFU, does simplex printing using first image forming apparatus  100  and merely conveys the sheet on second image forming apparatus  200 . Alternatively, image forming system  10  feeds a sheet from sheet feed unit PFU, merely conveys a sheet on first image forming apparatus  100  without printing, and does simplex printing using second image forming apparatus  200 . Now, a flow of printing will be described in detail using the former case as a representative case. First, image forming system  10  feeds a sheet from sheet feed unit PFU and prints on a front side of the sheet using first image forming apparatus  100 . Subsequently, image forming system  10  conveys the sheet to second image forming apparatus  200 . Then, image forming system  10  passes the simplex-printed sheet through second image forming apparatus  200 . When post-processing is needed, image forming system  10  makes post-processing apparatus  300  apply post-processing such as multi-folding, saddle stitching, or side stitching or processing such as reversal to the simplex-printed sheet. Finally, image forming system  10  ejects the simplex-printed sheet onto sheet output tray  310 . 
     When doing duplex printing on a normal sheet, the image forming system  10  feeds a sheet from sheet feed unit PFU and prints on a front side (upper side) of the sheet using first image forming apparatus  100 . Subsequently, image forming system  10  conveys the sheet to second image forming apparatus  200 . Then, image forming system  10  prints on a back side (lower side) of the sheet using second image forming apparatus  200 . After printing on the back side of the sheet, image forming system  10  makes reversing mechanism R 2  reverse the sheet and conveys the reversed sheet to post-processing apparatus  300 . When post-processing is needed, image forming system  10  makes post-processing apparatus  300  apply post-processing such as multi-folding, saddle stitching, or side stitching to the sheet. Finally, image forming system  10  ejects the duplex-printed or post-processed sheet onto sheet output tray  310 . 
     Also, image forming system  10  according to the present embodiment is designed to be able to print on a partially thickened chip-embedded sheet, and details of such a printing process will be described later. Hereinafter the term “sheet” when used without any modification can mean both a normal sheet and a chip-embedded sheet. 
     Next, a functional configuration of image forming system  10  will be described. As shown in  FIG. 2 , image forming system  10  is an MFP (Multi Function Peripheral) that includes first image forming apparatus  100  and second image forming apparatus  200 . Note that in  FIG. 2 , illustration of sheet feed unit PFU and post-processing apparatus  300  is omitted. 
     First image forming apparatus  100  includes control section  101 , document reading section  110 , operation/display section  120 , image processing section  130 , image forming section  140 , conveyance section  150 , fixing section  160  (first fixing section), communication section  171 , and storage section  172 . 
     Control section  101  includes CPU (Central Processing Unit)  102 , ROM (Read Only Memory)  103 , RAM (Random Access Memory)  104 , and the like. CPU  102  reads a program corresponding to processing details out of ROM  103 , loads the program into RAM  104 , and controls various blocks of first image forming apparatus  100  in collaboration with the loaded program. This is done by referring to various data stored in storage section  172 . Storage section  172  is made up, for example, of a non-volatile semiconductor memory (so-called flash memory) or a hard disk drive. 
     Control section  101  exchanges various data, via communication section  171 , with an external apparatus (e.g., a personal computer) connected to a communication network such as a LAN (Local Area Network) or WAN (Wide Area Network). Control section  101  receives, for example, image data transmitted from the external apparatus, and causes an image to be formed on a sheet based on the image data (input image data). Communication section  171  is made up, for example, of a communication control card such as a LAN card. 
     According to the present embodiment, control section  101  exchanges various data with second image forming apparatus  200  via communication section  171 . Also, control section  101  controls operation of second image forming apparatus  200  in conjunction with control section  201  of second image forming apparatus  200 . 
     Document reading section  110  optically scans a document conveyed onto contact glass, focuses light reflected from the document on a light-receiving surface of a CCD (Charge Coupled Device) sensor, and reads the document. Note that the document is conveyed onto the contact glass by an automatic document feeder (ADF), but the document may be put on the contact glass by manual operation. 
     Operation/display section  120  is made up, for example, of a liquid crystal display (LCD) with a touch panel and functions as a display section and an operation section. Based on a display control signal inputted from control section  101 , the display section displays various operation screens, states of images, operating status of each function, and the like. The operation section is equipped with various operation keys such as a numeric keypad and start key, accepts various input operations from a user, and outputs an operation signal to control section  101 . 
     Image processing section  130  includes a circuit adapted to perform an analog-digital (A/D) conversion process and a circuit adapted to perform digital image processing. Image processing section  130  generates digital image data by A/D conversion process from analog image signal acquired by a CCD sensor of document reading section  110  and outputs the digital image data to image forming section  140 . Also, image processing section  130  makes a tone correction based on tone correction data (tone correction table LUT) in storage section  172  under the control of control section  101 . Also, in addition to the tone correction, image processing section  130  applies various correction processes such as color correction and shading correction as well as a compression process to input image data. Image forming section  140  is controlled based on the image data subjected to these processes. 
     Based on the digital image data generated by image processing section  130 , image forming section  140  emits laser light, irradiates a photoconductor drum with the emitted laser light, and thereby forms an electrostatic latent image on the photoconductor drum (exposure step). 
     Image forming section  140  has a configuration for performing a charging step, the exposure step described above, a developing step, a transfer step, and a cleaning step in the order mentioned. 
     In the charging step, image forming section  140  uniformly charges a surface of the photoconductor drum using corona discharge from a charging apparatus. In the developing step, image forming section  140  deposits toner contained in a developer in a developing apparatus on the electrostatic latent image on the photoconductor drum and thereby forms a toner image. 
     In the transfer step, image forming section  140  transfers the toner image on the photoconductor drum onto the sheet conveyed by conveyance section  150 . In the cleaning step, image forming section  140  removes any toner remaining on the photoconductor drum after the transfer step. 
     Fixing section  160  applies heat and pressure to the toner image on the sheet introduced into the fixing nip section (thermal fixing) and thereby fixes the toner image on the sheet (fixing step). Consequently a fixed toner image is formed on the sheet. 
     Second image forming apparatus  200  includes control section  201 , image forming section  210 , conveyance section  220 , fixing section  230  (second fixing section), communication section  241 , and storage section  242 . Note that process of the components of second image forming apparatus  200  are similar, respectively, to processes of control section  101 , image forming section  140 , conveyance section  150 , fixing section  160 , communication section  171 , and storage section  172  of first image forming apparatus  100  described above, and thus description thereof will be omitted here. 
     [Configurations of Image Forming Sections  140  and  210  and Fixing Sections  160  and  230 ] 
     Next, configurations of image forming section  140  and fixing section  160  belonging to first image forming apparatus  100  as well as configurations of image forming section  210  and fixing section  230  belonging to second image forming apparatus  200  will be described with reference to  FIG. 3 . 
     Image forming section  140  of first image forming apparatus  100  includes photoconductor drum  141 , charging apparatus  142 , exposure apparatus  143 , developing apparatus  144 , transfer/conveyance path  145  adapted to lead the sheet to a transfer region, transfer belt  146  adapted to transfer the toner image formed on photoconductor drum  141  to the sheet, and cleaning apparatus  147  adapted to remove any toner remaining on photoconductor drum  141 . In image forming section  140 , charging apparatus  142 , exposure apparatus  143 , developing apparatus  144 , transfer/conveyance path  145 , transfer belt  146 , and cleaning apparatus  147  are provided along a rotational direction (direction of an arrow) of photoconductor drum  141 . Transfer/conveyance path  145  makes up part of conveyance section  150  shown in  FIG. 2 , and plural conveyance roller pairs  145   a  each made up mainly of a driving roller and driven roller are arranged on the transfer/conveyance path  145 . Conveyance roller pairs  145   a  are also placed on other conveyance paths of conveyance section  150 . 
     Transfer belt  146  is stretched between driven roller  148   a  and driving roller  148   b  and placed below photoconductor drum  141  such that a surface of transfer belt  146  will be in contact with part of an outer circumferential surface of photoconductor drum  141 . That is, transfer nip section NP serving as a transfer region is formed between transfer belt  146  and photoconductor drum  141 . The sheet is conveyed by being pressed against photoconductor drum  141  in transfer nip section NP by transfer belt  146 . 
     Transfer roller  149  capable of applying a transfer voltage to transfer belt  146  is placed on an inner side of transfer belt  146  put in contact with part of an outer circumferential surface of photoconductor drum  141 . Transfer roller  149  is connected with a voltage application section (not shown) serving as a power supply adapted to apply the transfer voltage to transfer belt  146 . Control section  101  controls the voltage to be applied by the voltage application section such that a predetermined current will flow to transfer belt  146  from transfer roller  149 . As the transfer voltage is applied to transfer belt  146 , the toner image on photoconductor drum  141  is transferred to the sheet placed in contact with photoconductor drum  141 . 
     Also, fixing section  160  is installed downstream of transfer belt  146  in a sheet conveyance direction. Fixing section  160  includes fixing roller  161  (first fixing member) kept at a predetermined heating temperature by a built-in heat source such as a halogen heater and pressure roller  162  (first pressing member) brought into pressing contact with fixing roller  161 . Fixing section  160  introduces the sheet into fixing nip section NP 1  (first fixing nip) between fixing roller  161  and pressure roller  162 , conveys the sheet by nipping the sheet, and thereby thermally fixes the unfixed toner image on the sheet using heat of fixing roller  161 . 
     In fixing section  160 , fixing roller  161  has, for example, a diameter of  70  mm and a rubber layer  6 . 5  mm thick on an outer circumferential side. An outer circumferential surface of the rubber layer is covered with a PFA tube resin layer for use as a surface release layer. 
     Pressure roller  162  is brought into pressing contact with fixing roller  161  under a predetermined fixing load. The fixing load can be adjusted by installing a known load varying mechanism (see Japanese Patent Application Laid-Open No. 2003-287932) equipped, for example, with a stepping motor, cam, and the like on pressure roller  162  and controlling the stepping motor by control section  101 . 
     Also, different values of the fixing load are used depending on the sheet type and basis weight of the sheet to be used. For example, in the case of a normal sheet uniform in sheet thickness, a fixing load of 700 N is used for a quality sheet and a plain sheet with a basis weight of less than 50 g/m 2 , a fixing load of 1100 N is used for a quality sheet with a basis weight of 50 to 74 g/m 2 , a fixing load of 1900 N is used for a plain sheet with a basis weight of 50 to 74 g/m 2 , and a fixing load of 1900 N is used for a quality sheet and a plain sheet with a basis weight of 75 g/m 2 . 
     Image forming section  210  of second image forming apparatus  200  includes photoconductor drum  211 , charging apparatus  212 , exposure apparatus  213 , developing apparatus  214 , transfer/conveyance path  215  adapted to lead the sheet to a transfer region, transfer belt  216  adapted to transfer the toner image formed on photoconductor drum  211  to the sheet, and cleaning apparatus  217  adapted to remove any toner remaining on photoconductor drum  211 . In image forming section  210  charging apparatus  212 , exposure apparatus  213 , developing apparatus  214 , transfer/conveyance path  215 , transfer belt  216 , and cleaning apparatus  217  are provided along a rotational direction (direction of an arrow) of photoconductor drum  211 . Transfer/conveyance path  215  makes up part of conveyance section  220  shown in  FIG. 2 , and plural conveyance roller pairs  215   a  each made up mainly of a driving roller and driven roller are arranged on the transfer/conveyance path  215 . Conveyance roller pairs  215   a  are also placed on other conveyance paths of conveyance section  220 . 
     Transfer belt  216  is stretched between driven roller  218   a  and driving roller  218   b  and placed below photoconductor drum  211  such that a surface of transfer belt  216  will be in contact with part of an outer circumferential surface of photoconductor drum  211 . That is, transfer nip section NP serving as a transfer region is formed between transfer belt  216  and photoconductor drum  211 . The sheet is conveyed by being pressed against photoconductor drum  211  in transfer nip section NP by transfer belt  216 . 
     Transfer roller  219  capable of applying a transfer voltage to transfer belt  216  is placed on an inner side of transfer belt  216  put in contact with part of an outer circumferential surface of photoconductor drum  211 . Transfer roller  219  is connected with a voltage application section (not shown) serving as a power supply adapted to apply the transfer voltage to transfer belt  216 . Control section  201  controls the voltage to be applied by the voltage application section such that a predetermined current will flow to transfer belt  216  from transfer roller  219 . As the transfer voltage is applied to transfer belt  216 , the toner image on photoconductor drum  211  is transferred to the sheet placed in contact with photoconductor drum  211 . 
     Also, fixing section  230  is installed downstream of transfer belt  216  in the sheet conveyance direction. Fixing section  230  includes fixing roller  231  (second fixing member) kept at a predetermined heating temperature by a built-in heat source such as a halogen heater and pressure roller  232  (second pressing member) brought into pressing contact with fixing roller  231 . Fixing section  230  introduces the sheet into fixing nip section NP 1  (second fixing nip) between fixing roller  231  and pressure roller  232 , conveys the sheet by nipping the sheet, and thereby thermally fixes the unfixed toner image on the sheet using heat of fixing roller  231 . 
     In fixing section  230 , pressure roller  232  is brought into pressing contact with fixing roller  231  under a predetermined fixing load. The fixing load can be adjusted using a mechanism and control similar to those described above. 
     Whereas in the present embodiment, a configuration of a heating roller type such as described above has been shown by example as fixing sections  160  and  230 , another configuration such as a heating belt type may be used alternatively. Also, the heat sources of fixing sections  160  and  230  may be an IH (induction heating) type. 
     Now, if a printing process similar to the one used for the above-mentioned normal sheet is used on image forming system  10  of the above configuration in forming images on a partially thickened sheet (sheet nonuniform in sheet thickness) such as a chip-embedded sheet rather than sheet uniform in sheet thickness, problems arise in terms of durability of fixing members during the fixing process. Now, a configuration of the chip-embedded sheet as well as problems encountered in forming images on the chip-embedded sheet will be described with reference to  FIG. 4 . 
     A cross-sectional shape of a chip-embedded sheet ICS is shown in  FIG. 4 . As shown in  FIG. 4 , chip-embedded sheet ICS is a recording material made up of two sheets S 1  and S 2  with IC chip C provided therebetween. Chip-embedded sheet ICS includes thin-sheet section A 1  as a first region containing no IC chip C and having a small sheet thickness and thick-sheet section A 2  as a second region containing IC chip C and having a large sheet thickness. An adhesive (not shown) is interposed between sheet S 1  and sheet S 2  of chip-embedded sheet ICS. 
     In chip-embedded sheet ICS, the sheet thickness of thin-sheet section A 1  is, for example, around 200 μm and the sheet thickness of thick-sheet section A 2  is, for example, around 500 μm. Depending on the thickness and the like of IC chip C, the sheet thickness of thick-sheet section A 2  in chip-embedded sheet ICS is generally 1.5 to 3 times the sheet thickness of thin-sheet section A 1 . 
     Regarding the configuration of chip-embedded sheet ICS, only a single IC chip C is shown in  FIG. 4 , but actually plural IC chips C may be arranged. In the case of chip-embedded sheet equipped with plural IC chips, the IC chips may be arranged in any desired form and may be equal to one another or different from one another in size or thickness. 
     With respect to chip-embedded sheet ICS, which is partially thickened sheet nonuniform in sheet thickness as described above, if fixing sections  160  and  230  perform a fixing process using normal fixing pressure (fixing load) used for a normal sheet uniform in sheet thickness, surface pressure will rise in a portion with a large sheet thickness, overloading fixing rollers  161  and  231  as well as pressure rollers  162  and  232  and thereby giving rise to a problem of reduced durability of the rollers. 
     As a means of dealing with the problem of durability, it is conceivable to perform the fixing process using a smaller fixing load than for a normal sheet. However, if the fixing load is reduced, although fixability can be ensured in thick-sheet section A 2  of chip-embedded sheet ICS by the nip surface pressure increased during passage of the sheet because of a large sheet thickness, fixability cannot be ensured in thin-sheet section A 1  because of low nip surface pressure. 
     To deal with this problem, according to the present embodiment, in printing a toner image on chip-embedded sheet ICS, that part of the image which is intended for thin-sheet section A 1  (first region) is formed and fixed by first image forming apparatus  100  and that part of the image which is intended for thick-sheet section A 2  (second region) is formed and fixed by second image forming apparatus  200 . Moreover, in printing a toner image on chip-embedded sheet ICS, respective fixing sections  160  and  230  of first and second image forming apparatuses  100  and  200  pass chip-embedded sheet ICS using a lower fixing load (second load) than the fixing load (first load) used for a sheet uniform in sheet thickness. 
     More specifically, in printing a toner image on chip-embedded sheet ICS, of the fixing loads, the second load is set to a value 25 to 45% lower than the first load. Also, in printing a toner image on chip-embedded sheet ICS, regardless of whether or not a toner image is formed, i.e., even when a toner image is formed only by either of first image forming apparatus  100  and second image forming apparatus  200 , the second load is used as the fixing load. Also, in printing a toner image on chip-embedded sheet ICS, a single-side mode in which an image is formed only on one side (upper side or lower side in  FIG. 4 ) of chip-embedded sheet ICS is established, disabling the use of reversal mechanism R 1  adapted to reverse the front and back sides of the sheet. 
     When forming images on a partially thickened sheet such as chip-embedded sheet ICS, this configuration makes it possible to ensure good fixability and ensure durability of members (fixing rollers  161  and  231  as well as pressure rollers  162  and  232 ) forming fixing nip NP 1 . 
     That is, according to the present embodiment, in printing a toner image on chip-embedded sheet ICS, first, a toner image is formed on thin-sheet section A 1  not containing IC chip C by image forming section  140  of first image forming apparatus  100 , which is an upstream apparatus, and the sheet is passed by setting the fixing load of fixing section  160  to the second load lower than normal. Next, a toner image is formed on thick-sheet section A 2  containing IC chip C by image forming section  210  of second image forming apparatus  200 , which is a downstream apparatus, and the sheet is passed by setting the fixing load of fixing section  230  to the second load lower than normal. 
     In this operation, regarding thick-sheet section A 2  containing IC chip C, even if the fixing load is the second load lower than normal, the nip surface pressure increases during passage of the sheet in fixing section  230  because of the large sheet thickness, making it possible to ensure fixability. On the other hand, regarding thin-sheet section A 1  having a small sheet thickness, after a toner image is formed by image forming section  140  of first image forming apparatus  100 , since the fixing process is performed twice by fixing section  160  and by fixing section  230  of second image forming apparatus  200 , fixability can be ensured even if the fixing load of both fixing sections  160  and  230  is the second load. 
     Therefore, the present embodiment can ensure good fixability and ensure the durability of the members forming fixing nip NP 1 , making it possible to combine good fixability with ensured durability of fixing rollers  161  and  231  and pressure rollers  162  and  232 . 
     Note that if the sequence of image formation is changed, i.e., if a toner image is formed on thick-sheet section A 2  by first image forming apparatus  100  (upstream apparatus) and a toner image is formed on thin-sheet section A 1  by second image forming apparatus  200  (downstream apparatus), the effect described above is no longer available. 
     Specifically, in this case, thick-sheet section A 2  is passed through fixing sections  160  and  230 , and the respective nip surface pressures of the fixing sections increase, thereby allowing fixability of the toner image to be ensured, but because thin-sheet section A 1  is passed through fixing section  230  only once after formation of the toner image, the fixability of the toner image cannot be ensured. Here, it is conceivable to set the fixing load of only fixing section  230  of the downstream apparatus higher than the second load. In that case, however, even though the fixability of the toner image on thin-sheet section A 1  can be ensured, the durability of fixing roller  231  and pressure roller  232  of fixing section  230  is reduced. 
     Therefore, the configuration described above makes it possible to combine good fixability of the toner image formed on thin-sheet section A 1  and thick-sheet section A 2  with ensured durability of fixing rollers  161  and  231  and pressure rollers  162  and  232 . 
     According to the present embodiment, when chip-embedded sheet ICS is used as a sheet, a non-illustrated sheet-profile setting screen is displayed on operation/display section  120 , allowing the user to register the sheet size of chip-embedded sheet ICS, the location of thick-sheet section A 2  on the sheet, and the like as user-defined information on the setting screen in advance. 
     A processing flow of a print job executed by image forming system  10  using chip-embedded sheet ICS will be described below with reference to a flowchart of  FIG. 5 . 
     In Step S 10 , control section  101  of first image forming apparatus  100  waits until a print job is received, and when a print job is received (YES in Step S 10 ), control section  101  goes to Step S 20 . 
     In Step S 20 , control section  101  determines with reference to details of the print job whether the sheet to be used is a chip-embedded sheet, and when the sheet to be used is a chip-embedded sheet (YES in Step S 20 ), control section  101  goes to Step S 30 . On the other hand, when the sheet to be used is not a chip-embedded sheet (NO in Step S 20 ), control section  101  skips to Step S 50 , regarding that the sheet to be used is a sheet uniform in sheet thickness. 
     In Step S 30 , control section  101  detects positions of thin-sheet section A 1  and thick-sheet section A 2  based on the user-defined information described above, where thin-sheet section A 1  and thick-sheet section A 2  are the first region and second region of a chip-embedded sheet to be used, respectively. 
     In Step S 40 , control section  101  sets the fixing load of respective fixing sections  160  and  230  of image forming apparatuses  100  and  200  to a low value (second load). The value of the second load is set 25 to 45% lower than a normal load, i.e., the first load used for a sheet uniform in sheet thickness as described above. According to the present embodiment, the second load set in Step S 40  is identical between fixing sections  160  and  230 . As another example of settings, the fixing loads used in respective fixing sections  160  and  230  may be set to values different from each other as long as the second load is lower than the first load described above. 
     In Step S 50 , control section  101  controls image forming sections  140  and  210 , fixing sections  160  and  230 , and the like such that image forming apparatuses  100  and  200  will perform respective printing processes. 
     Specifically, to print a toner image on chip-embedded sheet ICS in Step S 50 , by referring to input image data, control section  101  controls image forming section  140  and fixing section  160  such that first image forming apparatus  100  will print the toner image on thin-sheet section A 1 , which is the first region. Here, even if no toner image is formed on thin-sheet section A 1  by image forming section  140  (i.e., even if an image forming step is not carried out), a fixing step (i.e., heating under pressure exerted by the second load) is carried out by fixing section  160 . 
     Next, control section  101  performs control such that chip-embedded sheet ICS described above will be conveyed to second image forming apparatus  200  and controls image forming section  210  and fixing section  230  such that a toner image will be printed by second image forming apparatus  200  on thick-sheet section A 2 , which is the second region. Alternatively, control section  101  sends instructions to control section  201 , specifying a toner image to be printed on thick-sheet section A 2  by image forming section  210  and fixing section  230  of second image forming apparatus  200 . Here, even if no toner image is formed on thick-sheet section A 2  by image forming section  210  (i.e., even if an image forming step is not carried out), a fixing step (i.e., heating under pressure exerted by the second load) is carried out by fixing section  230 . 
     The above process makes it possible to ensure good fixability in forming an image on chip-embedded sheet ICS as well as ensure the durability of the fixing members (fixing rollers  161  and  231 , pressure rollers  162  and  232 , and the like) forming fixing nip NP 1 . 
     On the other hand, to print a toner image on a sheet other than chip-embedded sheet ICS in Step S 50 , based on the print job and input image data, control section  101  controls various part such that the input image will be printed and a sheet will be conveyed according to the print job. 
     Embodiment 2 
     Next, Embodiment 2 in which an image is formed on chip-embedded sheet ICS will be described with reference to  FIG. 6  to  FIG. 8 . 
       FIG. 6  schematically shows an overall configuration of image forming apparatus  1  according to Embodiment 2; and  FIG. 7  shows principal components of a control system of image forming apparatus  1 . For the sake of simplicity, components equal in configuration to first image forming apparatus  100  of image forming system  10  described above are denoted below by the same reference numerals as the corresponding components of first image forming apparatus  100 , and description thereof will be omitted as appropriate. 
     As shown in  FIG. 6 , image forming apparatus  1  is an intermediate-transfer color image forming apparatus using electrophotographic technology. That is, image forming apparatus  1  primarily transfers Y (yellow), M (magenta), C (cyan), and K (black) toner images formed on photoconductor drums  413  to intermediate transfer belt  421 , superimposes the toner images of four colors one on top of another on intermediate transfer belt  421 , then secondarily transfers the toner images onto sheet S sent out from sheet feed units  51   a  to  51   c,  and thereby forms an image. 
     Image forming apparatus  1  adopts a tandem system in which photoconductor drums  413  corresponding to the four YMCK colors are arranged in series along a running direction of intermediate transfer belt  421  and the images of the different colors are transferred to intermediate transfer belt  421  in sequence in a single procedure. 
     As shown in  FIG. 6  and  FIG. 7 , image forming apparatus  1  is an MFP (Multi Function Peripheral) that includes document reading section  110 , operation/display section  120 , image processing section  130 , image forming section  140 , sheet conveyance section  150 , fixing section  160 , control section  101 , communication section  171 , storage section  172 , and the like. 
     Document reading section  110  includes automatic document feeding apparatus  111  also known as ADF (Auto Document Feeder), original-image scanning apparatus  112  (scanner), and the like. 
     Automatic document feeding apparatus  111  conveys document D placed on a document tray using a conveyance mechanism and thereby delivers document D to original-image scanning apparatus  112 . Automatic document feeding apparatus  111  makes it possible to read a large number of documents D (including duplex mode) placed on the document tray, consecutively in one go. 
     Original-image scanning apparatus  112  optically scans a document conveyed onto the contact glass from automatic document feeding apparatus  111  or a document put on the contact glass, focuses light reflected from the document on the light-receiving surface of a CCD (Charge Coupled Device) sensor  112   a,  and reads an original image. Document reading section  110  generates input image data based on read results produced by original-image scanning apparatus  112 . The input image data undergoes predetermined image processing performed by image processing section  130 . 
     Operation/display section  120  is made up, for example, of a liquid crystal display with a touch panel and functions as display section  121  and operation section  122 . 
     Image processing section  130  includes a circuit and the like adapted to perform digital image processing on input image data according to default settings or user settings. 
     Image forming section  140  includes image forming units  141 Y,  141 M,  141 C, and  141 K adapted to form images using Y, M, C, and K color toners based on input image data as well as includes intermediate transfer unit  142  and the like. 
     The image forming units  141 Y,  141 M,  141 C, and  141 K for Y, M, C, and K colors have similar configurations. For convenience of illustration and explanation, similar components are denoted by the same reference numerals and Y, M, C, or K are added to the reference numerals only when it is necessary to distinguish among components for different colors. In  FIG. 6 , only the components of the image forming unit  141 Y for the Y color are denoted by reference numerals, and the reference numerals of the components of the other image forming units  141 M,  141 C, and  141 K are omitted. 
     Each image forming unit  141  includes exposure apparatus  411 , developing apparatus  412 , photoconductor drum  413 , charging apparatus  414 , drum cleaning apparatus  415 , and the like. 
     Photoconductor drum  413  is a negatively-charged organic photo-conductor (OPC) created by laminating an under coat layer (UCL), a charge generation layer (CGL), and a charge transport layer (CTL) in sequence on a circumferential surface of, for example, a conductive cylindrical aluminum body (aluminum pipe stock). The charge generation layer is made of an organic semiconductor produced by scattering a charge generation material (e.g., phthalocyanine pigment) in a resin binder (e.g., polycarbonate) and generates a pair of positive and negative charges when exposed by exposure apparatus  411 . The charge transport layer is produced by scattering a positive-hole transport material (electron-donating nitrogen-containing compound) in a resin binder (e.g., polycarbonate resin) and conveys positive charges generated in the charge generation layer to a surface of the charge transport layer. 
     Control section  101  controls a driving current supplied to a drive motor (not shown) adapted to rotate each photoconductor drum  413 , and thereby rotates the photoconductor drum  413  at a constant circumferential speed (linear velocity). 
     Charging apparatuses  414  impart uniformly negative charges to surfaces of respective photoconductor drums  413  having photoconductivity. Exposure apparatuses  411  are made up, for example, of semiconductor lasers and irradiate photoconductor drums  413  with laser beams corresponding to respective color components of the image. A positive charge is generated in the charge generation layer of each photoconductor drum  413 , conveyed to the surface of the charge transport layer, thereby neutralizing a surface charge (negative charge) of photoconductor drum  413 . An electrostatic latent image of the corresponding color component is formed on the surface of each photoconductor drum  413  due to a potential difference from surroundings. 
     Developing apparatuses  412 , which are, for example, two-component developing apparatuses, deposit toners of respective colors on the surfaces of photoconductor drums  413 , thereby visualize electrostatic latent images, and form toner images. 
     Each drum cleaning apparatus  415  includes a cleaning blade and the like, where the cleaning blade is placed in sliding contact with the surface of photoconductor drum  413 . Drum cleaning apparatus  415  removes residual toner remaining on the surface of photoconductor drum  413  after primary transfer using the cleaning blade. 
     Intermediate transfer unit  142  includes intermediate transfer belt  421  serving as an image carrier, primary transfer roller  422 , plural support rollers  423 , secondary transfer roller  424 , belt cleaning apparatus  426 , and the like. 
     Intermediate transfer belt  421  is an endless belt and is stretched over plural support rollers  423 , forming a loop. At least one of plural support rollers  423  is a driving roller, and the other rollers are driven rollers. Preferably, for example, roller  423 A placed downstream of primary transfer roller  422  for the K color in a belt running direction is a driving roller. This makes it easy to keep belt running speed in a primary transfer section constant. As driving roller  423 A rotates, intermediate transfer belt  421  runs at a constant speed in the direction of arrow A. 
     Primary transfer roller  422  is placed on an inner circumferential side of intermediate transfer belt  421 , facing photoconductor drums  413  for the respective colors. Primary transfer roller  422  is brought into pressing contact with photoconductor drums  413  with intermediate transfer belt  421  pinched therebetween, thereby forming a primary transfer nip used to transfer toner images from photoconductor drums  413  to intermediate transfer belt  421 . 
     Secondary transfer roller  424  is placed on an outer circumferential side of intermediate transfer belt  421 , facing backup roller  423 B placed downstream of driving roller  423 A in the belt running direction. Secondary transfer roller  424  is brought into pressing contact with backup roller  423 B with intermediate transfer belt  421  pinched therebetween, thereby forming a secondary transfer nip used to transfer toner images from intermediate transfer belt  421  to the sheet. 
     When intermediate transfer belt  421  passes through the primary transfer nip, toner images on photoconductor drums  413  are primarily transferred in sequence onto intermediate transfer belt  421 , being superimposed one on top of another. Specifically, primary transfer bias is applied to primary transfer roller  422  and a charge opposite in polarity to the toner is applied to a back side of intermediate transfer belt  421  (side on which intermediate transfer belt  421  abuts primary transfer roller  422 ), thereby electrostatically transferring the toner images onto intermediate transfer belt  421 . 
     Subsequently, when the sheet passes through the secondary transfer nip, the toner images on intermediate transfer belt  421  are secondarily transferred to the sheet. Specifically, secondary transfer bias is applied to secondary transfer roller  424  and a charge opposite in polarity to the toner is applied to a back side of the sheet (side on which the sheet abuts secondary transfer roller  424 ), thereby electrostatically transferring the toner images onto the sheet. The sheet with the toner images transferred thereto is conveyed toward fixing section  160 . 
     Belt cleaning apparatus  426  includes a belt cleaning blade and the like, the belt cleaning blade being placed in sliding contact with a surface of intermediate transfer belt  421 , and removes residual toner remaining on the surface of intermediate transfer belt  421  after secondary transfer. Note that instead of secondary transfer roller  424 , a configuration (so-called belt-type secondary transfer unit) may be adopted in which an intermediate transfer belt is stretched in a loop over plural support rollers including a secondary transfer roller. 
     Fixing section  160  includes upper fixing section  60 A provided with a fixing-surface-side member placed on the side of a fixing surface (surface on which toner images are formed) of the sheet, lower fixing section  60 B provided with a back-side support member placed on the back side (side opposite the fixing surface) of the sheet, heating source  60 C, and the like. The back-side support member is brought into pressing contact with the fixing-surface-side member, thereby forming a fixing nip configured to nip and convey the sheet. 
     With the toner images secondarily transferred, fixing section  160  heats and pressurizes the incoming sheet in the fixing nip and thereby fixes the toner images on the sheet. The fixing section  160  is installed as a unit in fuser F. Also, pneumatic separation unit  60 D adapted to separate sheet S from the fixing-surface-side member by blowing air is installed in fuser F. 
     Upper fixing section  60 A includes endless fixing belt  61 , which is a fixing-surface-side member, heating roller  62 , and upper pressure roller  63  (belt heating type). Fixing belt  61  is stretched over heating roller  62  and upper pressure roller  63  at a predetermined tension (e.g., 400 N). 
     Fixing belt  61  is produced by covering an outer circumferential surface of a base made, for example, of PI (polyimide) with a heat-resistant silicon rubber for use as an elastic layer and further covering or coating a surface layer with a tube of PFA (perfluoroalkoxy) which is a heat-resistant resin. 
     Fixing belt  61  comes into contact with the sheet on which toner images have been formed and thermally fixes the toner images on the sheet within a permissible fixing-temperature range. Here, a permissible fixing-temperature range is a temperature range in which an amount of heat needed to melt the toner on the sheet can be supplied and varies with the type of sheet used for image formation and the like. 
     Heating roller  62  heats fixing belt  61 . Heating roller  62  incorporates heating source  60 C adapted to heat fixing belt  61 . Heating roller  62  is, for example, a halogen heater and is covered with a resin layer produced by coating an outer circumferential surface of a cylindrical metal core formed of aluminum or the like with PTFE. 
     The temperature of heating source  60 C is controlled by control section  101 . Heating roller  62  is heated by heating source  60 C, consequently heating fixing belt  61 . 
     Upper pressure roller  63  is produced by covering a solid metal core formed, for example, of metal such as iron with a heat-resistant silicon rubber for use as an elastic layer and further with a resin layer coated with PTFE, which is a low-friction, heat-resistant resin. Upper pressure roller  63  is brought into pressing contact with lower pressure roller  65  via fixing belt  61 , where lower pressure roller  65  is driven and rotated by main drive source (not shown) in fixing section  160 . 
     Lower fixing section  60 B includes lower pressure roller  65 , which is, for example, a back-side support member (roller pressurizing type). Lower pressure roller  65  is produced by covering an outer circumferential surface of a base material layer made, for example, of PI (polyimide) with a heat-resistant silicon rubber for use as an elastic layer and further covering an outer circumferential surface of the elastic layer with a PFA tube resin layer for use as a surface release layer. 
     By controlling the main drive source (drive motor), control section  101  rotates lower pressure roller  65  in a counterclockwise direction in  FIG. 6 . The drive control (e.g., on/off of rotation, circumferential speed, etc.) of the drive motor is performed by control section  101 . 
     Lower pressure roller  65  incorporates heating source  65 A such as a halogen heater. When heating source  65 A generates heat, lower pressure roller  65  is heated. Control section  101  controls electric power supplied to heating source  65 A and thereby keeps lower pressure roller  65  at a predetermined temperature. 
     Lower pressure roller  65  is brought into pressing contact with upper pressure roller  63  under a predetermined fixing load via fixing belt  61 . The fixing load can be adjusted in the same manner as described above by installing a known load varying mechanism equipped, for example, with a stepping motor, cam, and the like on lower pressure roller  65  and controlling the stepping motor by control section  101 . In this way, fixing nip NP 1  configured to nip and convey the sheet is formed between upper pressure roller  63  and lower pressure roller  65  via fixing belt  61 . 
     Conveyance section  150  includes sheet feed section  151 , sheet output section  152 , conveyance path segment  153 , and the like. Sheet S (standard sheet, special sheet) identified based on the basis weight (stiffness), size, and the like are housed in three sheet feed units  51   a  to  51   c  of sheet feed section  151  by being sorted according to types set in advance. Conveyance path segment  153  includes plural conveyance rollers including a resist roller pair  153   a,  a duplex conveyance path used to form images on both sides of a sheet, and the like. Note that details of conveyance path segment  153  will be described later. 
     In a simplex print mode for printing on one side of a sheet, sheet S stored in sheet feed units  151   a  to  151   c  is sent out sheet by sheet from the top and is conveyed to image forming section  140  by conveyance path segment  153 . In so doing, any slant of fed sheet S is corrected and conveyance timing is adjusted by a resist roller section in which resist roller pair  153   a  is disposed. Then, in image forming section  140 , the toner images on intermediate transfer belt  421  are secondarily transferred all together to one side of sheet S, and a fixing step is carried out by fixing section  160 . Sheet S with the image formed thereon is ejected out of the apparatus by sheet output section  152  equipped with sheet output rollers  52   a.    
     On the other hand, when printing toner images on chip-embedded sheet ICS described above, as shown in  FIG. 6 , chip-embedded sheet ICS is fed through external sheet feed slot  2   a  in apparatus body  2 . Also, when printing toner images on chip-embedded sheet ICS, control section  101  performs control such that chip-embedded sheet ICS will be passed through image forming section  140  and fixing section  160  in simplex print mode twice. Details of this control will be described later. 
     Next, conveyance path segment  153  will be described. 
     Conveyance path segment  153  is a path along which sheet is conveyed when an image is formed on one side, and includes a main conveyance path  530  along which the sheet used for image formation in image forming section  140  is conveyed. Main conveyance path  530  conveys the sheet through resist roller pair  153   a,  the secondary transfer nip of image forming section  140 , and fixing section  160 . Also, conveyance path segment  153  includes circulation conveyance path  533  to convey the sheet on which toner images have been printed to image forming section  140  again. 
     Conveyance path segment  153  includes external sheet feed/conveyance path  531  adapted to convey a sheet such as chip-embedded sheet ICS fed through external sheet feed slot  2   a  to main conveyance path  530 , sheet feed/conveyance path  532  adapted to convey sheet S fed from sheet feed units  151   a  to  151   c  to main conveyance path  530 . 
     Main conveyance path  530  is installed above sheet feed units  151   a  to  151   c  in apparatus body  2 , extending from one lateral side to another lateral side of apparatus body  2 . Main conveyance path  530  is connected at a first end to external sheet feed/conveyance path  531  and sheet feed/conveyance path  532 . Main conveyance path  530  is connected at a second end to an ejection slot of sheet output section  152  provided in the other lateral side of apparatus body  2 . 
     External sheet feed/conveyance path  531  is connected at a first end to external sheet feed slot  2   a  and connected at a second end to main conveyance path  530 . Sheet feed/conveyance path  532  is installed near one lateral side in apparatus body  2 , extending in an up-and-down (substantially vertical) direction from sheet feed units  151   a  to  151   c  to main conveyance path  530 . Sheet feed/conveyance path  532  is connected at an upper end to main conveyance path  530  and connected at a lower end to sheet feed units  151   a  to  151   c.    
     As shown in  FIG. 6 , circulation conveyance path  533  is a substantially C-shaped conveyance path installed between sheet feed units  151   a  to  151   c  and main conveyance path  530  in apparatus body  2  and provided with plural roller pairs for use to convey the sheet. 
     On an upstream side in the sheet conveyance direction, circulation conveyance path  533  includes branch conveyance path  533   a  branching off from main conveyance path  530 . In the conveyance direction of the sheet conveyed on main conveyance path  530 , branch conveyance path  533   a  is installed on a downstream side of fixing section  160 , branching downward from main conveyance path  530 . 
     On the most downstream side in the sheet conveyance direction, circulation conveyance path  533  includes merging conveyance path  533   b  merging into main conveyance path  530 . Merging conveyance path  533   b  is installed so as to merge into main conveyance path  530  at a location upstream of the secondary transfer nip of image forming section  140 . 
     Image forming apparatus  1  equipped with circulation conveyance path  533  described above can print toner images multiple times on the same side of the sheet by conveying the sheet on which toner images have been printed by image forming section  140  and fixing section  160  to circulation conveyance path  533 . 
     Next, the process of printing toner images on chip-embedded sheet ICS will be described with reference to a flowchart of  FIG. 8 . 
     In Step S 130 , control section  101  detects positions of thin-sheet section A 1  and thick-sheet section A 2  based on the user-defined information described above, where thin-sheet section A 1  and thick-sheet section A 2  are the first region and second region of chip-embedded sheet ICS to be used, respectively. 
     In Step S 140 , control section  101  sets the fixing load of fixing section  160  to a low value (second load). The value of the second load is set 25 to 45% lower than a normal load, i.e., the first load used for a sheet uniform in sheet thickness as described above. 
     In Step S 150 , control section  101  controls image forming section  140 , fixing section  160 , and the like such that a first printing process will be performed on chip-embedded sheet ICS. Specifically, by referring to input image data, control section  101  controls image forming section  140  and fixing section  160  such that toner images will be printed on thin-sheet section A 1 , which is the first region. Here, even if no toner image is formed on thin-sheet section A 1  by image forming section  140  (i.e., even if an image forming step is not carried out), a fixing step (i.e., heating under pressure exerted by the second load) is carried out by fixing section  160 . 
     In Step S 160 , control section  101  performs conveyance control such that chip-embedded sheet ICS subjected to the first printing process will be conveyed to circulation conveyance path  533 . As a result of this control, chip-embedded sheet ICS is sent from fixing section  160  to circulation conveyance path  533  through branch conveyance path  533   a  and fed again from merging conveyance path  533   b  to image forming section  140  through main conveyance path  530 . 
     In Step S 170 , control section  101  controls image forming section  140 , fixing section  160 , and the like such that a second printing process will be performed on chip-embedded sheet ICS. Specifically, by referring to the input image data, control section  101  controls image forming section  140  and fixing section  160  such that toner images will be printed on thick-sheet section A 2 , which is the second region. Here, even if no toner image is formed on thick-sheet section A 2  by image forming section  140  (i.e., even if an image forming step is not carried out), a fixing step (i.e., heating under pressure exerted by the second load) is carried out by fixing section  160 . 
     According to the present embodiment, the fixing load applied to fixing section  160  in the second printing process is equal to the fixing load applied to fixing section  160  in the first printing process. As another example, the fixing loads used in the first and second printing processes may differ from each other as long as the second load is lower than the first load described above. 
     Next, control section  101  performs conveyance control such that chip-embedded sheet ICS subjected to the second printing process will be ejected out of the apparatus through sheet output section  152 . 
     The present embodiment, which performs the control described above, can ensure good fixability in forming an image on chip-embedded sheet ICS as well as ensure the durability of the fixing members (fixing belt  61 , upper pressure roller  63 , lower pressure roller  65 , and the like) forming the fixing nip. 
     According to the present embodiment, when the first printing process on chip-embedded sheet ICS is finished, control section  101  performs conveyance control of chip-embedded sheet ICS such that chip-embedded sheet ICS will be conveyed for the second printing process to circulation conveyance path  533 . As another example, when the first printing process on chip-embedded sheet ICS is finished, control section  101  may perform conveyance control such that chip-embedded sheet ICS will be ejected out of the apparatus through sheet output section  152  and then fed manually into external sheet feed slot  2   a  by the user to perform the second printing process. 
     Embodiment 3 
     Next, Embodiment 3 in which an image is formed on chip-embedded sheet ICS will be described with reference to  FIG. 9  and  FIG. 10 . 
       FIG. 9  is diagram schematically showing an overall configuration of a network system according to Embodiment  3 . The network system shown in  FIG. 9  is connected with plural MFPs (Multi Function Peripherals) and server SA via network  811  such as the Internet, a LAN, or a communication network, where server SA centrally controls plural MFPs (MFP 1  to MFPn). Then, a print job transmitted from an external apparatus such as a PC is received by server SA, and thus can be executed by any of the MFPs (destination) through server SA. 
     Here, as a printing apparatus adapted to form images on a sheet, MFP includes the series-tandem image forming system described above (hereinafter referred to as a tandem machine) and the stand-alone image forming apparatus described above (hereinafter referred to as a single-machine). Also, a job management program is stored in a memory (recording medium) in server SA. By reading and executing the job management program, server SA assigns print jobs to pre-registered MFPs (MFP 1  to MFPn), making any of the MFPs execute the print jobs. 
     A process performed by server SA based on the job management program when a print job of forming an image on chip-embedded sheet ICS is received by server SA will be described below with reference to  FIG. 10 . 
     In Step S 210 , by referring to data on the received print job, server SA verifies the size, first region (thin-sheet section A 1 ), and second region (thick-sheet section A 2 ) of chip-embedded sheet ICS to be used and temporarily stores the data on chip-embedded sheet ICS in the memory and the like. 
     In Step S 220 , of the MFPs registered on the network, server SA extracts all the MFP models compatible with chip-embedded sheet ICS to be used. 
     In Step S 230 , server SA detects the states (operating status) of the extracted models (MFPs) and determines, in Step S 240  next, whether there is any tandem machine that is not busy. When it is determined that there is a non-busy tandem machine (YES in Step S 240 ), server SA goes to Step S 250  and sends data, instructions, and the like concerning the print job to the non-busy tandem machine. Note that if there are two or more non-busy tandem machines, server SA selects the tandem machine that can process the print job most quickly and transmits the data, instructions, and the like concerning the print job. 
     Here, the data concerning the print job includes the input image data as well as the data on chip-embedded sheet ICS temporarily stored in Step S 210 . The data on instructions concerning the print job includes an instruction to form and fix the image in the first region (thin-sheet section A 1 ) in the first printing process (on the first, upstream apparatus) and an instruction to form and fix the image in the second region (thick-sheet section A 2 ) in the second printing process (on the second, downstream apparatus). Also, the data concerning the print job may include an instruction to do fixing using a fixing load 25 to 45% lower than normal in the printing processes. 
     On the other hand, when it is determined that there is no non-busy tandem machine (NO in Step S 240 ), server SA goes to Step S 260  and determines whether there is any single-machine that is not busy. When it is determined that there is a non-busy single-machine (YES in Step S 260 ), server SA goes to Step S 270  and sends data, instructions, and the like concerning the print job to the non-busy single-machine. Note that if there are two or more non-busy single-machines, server SA selects the single-machine that can process the print job most quickly and transmits the data, instructions, and the like concerning the print job. 
     Here, as with the above case, the data concerning the print job includes the input image data as well as the data on chip-embedded sheet ICS temporarily stored in Step S 210 . Also, the data concerning the print job includes an instruction to form and fix the image in the first region (thin-sheet section) in the first printing process and an instruction to form and fix the image in the second region (thick-sheet section) in the second printing process. Also, the data concerning the print job may include an instruction to do fixing using a fixing load 25 to 45% lower than normal in the printing processes. 
     On the other hand, when it is determined that there is no non-busy single-machine (NO in Step S 260 ), server SA returns to Step S 230  and waits until any tandem machine or single-machine becomes free among the MFPs compatible with chip-embedded sheet ICS to be used. 
     In this way, in the present network system, upon receiving a print job of printing on chip-embedded sheet ICS, server SA specifies an MFP for use in the print job by taking the operating status of the registered MFPs into consideration, and sends instructions such as described above to the specified MFP. This configuration ensures both good fixability and durability of the members forming the fixing nip on the MFP instructed to execute the print job. 
     Also, in the present network system, in relation to the print job of printing on chip-embedded sheet ICS, since the job is assigned preferentially to a non-busy tandem machine, productivity is improved in printing toner images on chip-embedded sheet ICS. 
     In the embodiment described above, chip-embedded sheet ICS made up of two sheets (S 1  and S 2 ) with an IC chip sandwiched therebetween has been shown as an example of a sheet nonuniform in sheet thickness, the nonuniform sheet being made up of the first region of small sheet thickness and second region of large sheet thickness. However, the sheet nonuniform in sheet thickness is not limited in this, and other types of sheet can effectively be used in the present invention, where examples of the other types of sheet include a sheet in which any of various objects such as an electronic music box, pressed flower, and circuit pattern is sandwiched between two sheets S 1  and S 2  and the sandwiching portion (second region) is 1.5 to 3 times as thick as the other portion (first region) smaller in sheet thickness. 
     Although embodiments of the present invention have been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and not limitation, the scope of the present invention should be interpreted by terms of the appended claims.