Patent Publication Number: US-9405236-B2

Title: Image forming device

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on an application No. 2014-123596 filed in Japan, the contents of which are hereby incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     (1) Field of the Invention 
     The present disclosure pertains to an image forming device capable of both-side printing, such as a printer or a photocopier, executing a printing process by statically transferring an unfixed image onto a recording sheet and then thermally fixing the unfixed image onto the recording sheet. Particularly, the present disclosure pertains to a technology for controlling transfer voltage applied for the static transfer of the unfixed image during both-side printing. 
     (2) Description of the Related Art 
     An image forming device such as a printer, a photocopier, and the like is commonly provided with a both-side printing function of statically transferring an unfixed image to each of a first side (one side, e.g., a front side) of a recording sheet and a second side (the other side, e.g., a back side) of the recording sheet and then thermally fixing the unfixed images onto the recording sheet. 
     Also, a fixing temperature required for thermally fixing an unfixed image onto a recording sheet varies under different conditions, such as an amount of toner to be fixed onto the recording sheet, a type of image to be formed on the recording sheet, and the like. In order to prevent insufficient fixing under these different conditions, a target fixing temperature at which a surface temperature of a heating rotating body is to be maintained during thermal fixing with respect to a recording sheet page may be set to a temperature sufficient to ensure good fixing under printing conditions requiring the greatest amount of heat to be applied for the thermal fixing. 
     However, when the target fixing temperature is set as described above, more electricity than necessary is consumed particularly when thermally fixing an unfixed image on a page that does not require the greatest amount of heat, which is not desirable for energy conservation. 
     Technology for reducing the electricity consumption of thermal fixing has been proposed, such as Patent Literature 1 (Japanese Patent Application No. 2012-118496), which discloses changing the target fixing temperature at which the surface temperature of the heating rotating body is maintained during thermal fixing for each page, in accordance with the image content of the respective page. This enables adjusting the fixing temperature to an optimal temperature that is in accordance with the image content of a page that prevents the fixing temperature applied from becoming excessive or insufficient, which in turn reduces the electricity consumption required for thermal fixing. 
     As described above, the technology described by Patent Literature 1 varies the target fixing temperature between pages. Due to this, particularly when continuously printing two or more pages, the fixing temperature may change by a great amount while performing thermal fixing with respect to one page. In such circumstances, when the two or more pages are two sides of one recording sheet with respect to which both-side printing is performed, the amount of water contained (water content) in the recording sheet may vary in a sheet passing direction due to the change in fixing temperature occurring while thermal fixing is performed with respect to one side. As a result, electrical resistance in the sheet passing direction changes, which produces transfer unevenness when statically transferring an unfixed image onto the other side. This results in deterioration of image quality of the other side. 
     In consideration of the above-described problem, the present disclosure aims to provide an image forming device having a both-side printing function enabling, during both-side printing, prevention of transfer unevenness caused by change in water content in a recording sheet. 
     SUMMARY OF THE INVENTION 
     Problems to be Solved by the Invention 
     In order to solve the above-described problem, one aspect of the present disclosure is an image forming device capable of performing both-side printing with respect to a recording sheet, the image forming device statically transferring, by application of a transfer voltage, an unfixed image formed on an image carrier to the recording sheet when passing through a transfer position, and then thermally fixing the unfixed image onto the recording sheet when the recording sheet passes through a fixing position where a heating rotating body is disposed, the image forming device including: a water content index acquisition unit configured to acquire an index value of a water content at each of a plurality of sheet-passing-direction positions of the recording sheet having undergone thermal fixing of a first unfixed image statically transferred onto a first side thereof; and a transfer control unit configured to control, for each of the positions of the recording sheet, a transfer voltage applied for statically transferring a second unfixed image onto a second side of the recording sheet, so that the lower the water content indexed by the index value of the position, the greater an absolute value of the transfer voltage. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       These and the other objects, advantages and features of the disclosure will become apparent from the following description thereof taken in conjunction with the accompanying drawings which illustrate a specific embodiment of the disclosure. 
       In the drawings: 
         FIG. 1  depicts the configuration of an image forming device  1 ; 
         FIG. 2  depicts the configuration of a control unit  60  and the relationship between the control unit  60  and main components subject to control; 
         FIG. 3  schematically describes the relationship between sampling sheet positions and fixing position temperatures; 
         FIG. 4  is a table indicating the relationship between the sampling sheet positions illustrated in  FIG. 3 , elapsed time from when a leading edge of a recording sheet P illustrated in  FIG. 3  reaches a fixing position, and fixing position temperatures pertaining to the sampling sheet positions; 
         FIG. 5  is a flowchart indicating operations of a fixing temperature control process performed by the control unit  60 ; 
         FIG. 6  is a flowchart indicating operations of a target temperature setting process; 
         FIG. 7  is a flowchart indicating operations of an inter-page fixing temperature adjustment process; 
         FIG. 8  is a flowchart indicating operations of a both-side printing transfer voltage control process performed by the control unit  60 ; 
         FIG. 9  depicts a specific example of an output transfer voltage table; 
         FIG. 10  is a graph describing the relationship between the sampling sheet positions, the fixing position temperatures, and applied transfer voltages in the output transfer voltage table; 
         FIG. 11  is a flowchart indicating operations of a sheet passing direction fixing temperature distribution sampling process; 
         FIG. 12  is a flowchart indicating operations of a transfer voltage control process; 
         FIG. 13  schematically illustrates how applied transfer voltage that is output is switched each time one of the sampling sheet positions reaches secondary transfer position  46 ; 
         FIG. 14  depicts a modification of the image forming device in  FIG. 1 ; 
         FIG. 15  depicts the configuration of a de-curling mechanism  92 ; 
         FIG. 16  is a flowchart indicating operations of a one-side printing curl control process performed by the control unit  60 ; 
         FIG. 17  is a flowchart indicating operations of a sheet passing direction fixing temperature change detection process; 
         FIG. 18  depicts another modification of the image forming device in  FIG. 1 ; 
         FIG. 19  depicts the configuration of a humidifier  97 ; and 
         FIG. 20  is a flowchart indicating a modification of the operations of the one-side printing curl control process in  FIG. 16 . 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     A tandem image forming device (hereinafter simply termed an image forming device) is described below as an example of an embodiment of the image forming device pertaining to one aspect of the disclosure. 
     [1] Image Forming Device Configuration 
     The configuration of an image forming device  1  pertaining to the present embodiment is described first.  FIG. 1  depicts the configuration of the image forming device  1  pertaining to the present embodiment. As depicted in  FIG. 1 , the image forming device  1  includes an image processing unit  3 , a feed unit  4 , a fixing device  5 , and a control unit  60 . 
     The image forming device  1  is connected to a network (e.g., a LAN) and, upon receiving a print instruction from an external terminal device (not diagrammed) or an operation panel having a non-diagrammed display unit, forms a toner image in each of yellow, magenta, cyan, and black in accordance with the instruction and then forms a full color image by overlay transfer of the toner images onto a recording sheet, thus realizing a printing process onto the recording sheet. The reproduction colors yellow, magenta, cyan, and black are hereinafter represented by the initials Y, M, C, and K. The reference signs for components pertaining to the respective reproduction colors have the initials Y, M, C, and K appended thereto. 
     The image processing unit  3  includes imaging units  3 Y,  3 M,  3 C, and  3 K, an intermediate transfer belt  11 , primary transfer rollers  35 Y,  35 M,  35 C, and  35 K, and a secondary transfer roller  47 . The imaging units  3 Y,  3 M,  3 C, and  3 K are each configured similarly. As such, the configuration of imaging unit  3 Y is described below as a representative example. 
     Imaging unit  3 Y ( 3 M,  3 C,  3 K) includes a photosensitive drum  31 Y ( 31 M,  31 C,  31 K), as well as a developing unit  32 Y ( 32 M,  32 C,  32 K), a charging unit  33 Y ( 33 M,  33 C,  33 K), a cleaner  34 Y ( 34 M,  34 C,  34 K) cleaning the photosensitive drum  31 Y ( 31 M,  31 C,  31 K), and an exposure unit  10 Y ( 10 M,  10 C,  10 K) disposed around the photosensitive drum  31 Y ( 31 M,  31 C,  31 K). A yellow Y toner image is created over the photosensitive drum  31 Y. The developing unit  32 Y ( 32 M,  32 C,  32 K) faces the photosensitive drum  31 Y ( 31 M,  31 C,  31 K) and transports charged toner to the photosensitive drum  31 Y ( 31 M,  31 C,  31 K). The intermediate transfer belt  11  is an endless belt suspended across a driving roller  12  and a driven roller  13 , and is driven to circulate in the direction indicated by arrow C. The exposure unit  10 Y ( 10 M,  10 C,  10 K) includes a light-emitting element such as a laser diode, emits a laser light for image formation in accordance with a drive signal from the control unit  60 , and performs an exposure scan of the photosensitive drum  31 Y ( 31 M,  31 C,  31 K). The exposure scan forms a latent static image on the photosensitive drum  31 Y ( 31 M,  31 C,  31 K) that has been charged by the charging unit  33 Y ( 33 M,  33 C,  33 K). Imaging units  3 M,  3 C, and  3 K also have latent static images similarly formed on the respective photosensitive drums  31 M,  31 C, and  31 K. 
     The latent static images respectively formed on each of the photosensitive drums (i.e., photosensitive drums  31 Y,  31 M,  31 C, and  31 K) are developed by respective developing units (i.e., developing units  32 Y,  32 M,  32 C, and  32 K) of the imaging units  3 Y,  3 M,  3 C, and  3 K, thus forming toner images (i.e., unfixed images) in each corresponding color on the photosensitive drums  31 Y,  31 M,  31 C, and  31 K. The unfixed images thus formed sequentially undergo a primary transfer onto the intermediate transfer belt  11  with timing offset so that each unfixed image is transferred to the same overlapping position on the intermediate transfer belt  11  performed by respective primary transfer rollers (i.e., primary transfer rollers  35 Y,  35 M,  35 C, and  35 K) corresponding to the imaging units  3 Y,  3 M,  3 C, and  3 K. Afterward, the unfixed images on the intermediate transfer belt  11  undergo a secondary transfer (also termed a static transfer) onto the recording sheet, performed all at once through the effect of static electricity from the secondary transfer roller  47 . A transfer voltage is applied to the secondary transfer roller  47  through control by the control unit  60 , the transfer voltage having opposite polarity to the toner (here, for example, the toner polarity is taken to be negative). 
     The recording sheet having the unfixed images having undergone the static transfer is in turn transported to the fixing device  5 , where the unfixed images on the recording sheet are thermally fixed onto the recording sheet through the application of heat and pressure by the fixing device  5 . 
     For one-side printing, the recording sheet is expelled from the image forming device  1  by an exit roller  71 , after the thermal fixing. For both-side printing, the recording sheet having undergone thermal fixing on one side (here, a front side for example) is transported by the exit roller  71 , then transported from the exit roller  71  along a reverse transport channel  75  via transport rollers  73 ,  74 ,  76 , and  77 , and then transported to a later-described timing roller  45  while flipped from back to front. This change of transport channel is performed by a channel switching member  72 . The operations of the channel switching member  72  are controlled by the control unit  60 . 
     Subsequently, the recording sheet is transported to a secondary transfer position  46  by the timing roller  45 , an unfixed image is statically transferred onto the other side (here, a back side for example) of the recording sheet by the secondary transfer roller  47 , and the recording sheet is expelled from the image forming device  1  by the exit roller  71  after thermal fixing by the fixing device  5 . 
     Accordingly, it is possible to statically transfer and heat-fix an unfixed image onto the other side of the recording sheet, onto which an unfixed image has not been statically transferred at the point when the one-side printing is completed. 
     The feed unit  4  includes a paper feed cassette  41  containing the recording sheet, represented by the symbol P, a feed roller  42  feeding the recording sheet in the paper feed cassette  41  one at a time onto a transport channel  44 , a transport roller  43  transporting the recording sheet, once fed, to the timing roller  45 , and the timing roller  45  transporting the recording sheet, once transported, to the secondary transfer position  46  with transmission timing. A sheet passing sensor  81  is provided along the transport channel  44  between the timing roller  45  and the secondary transfer position  46 , and detects passing of the recording sheet. 
     The paper feed cassette  41  is not limited to being singular, and may also be provided in plurality. The recording sheet may be provided as a plurality of varieties of paper differing in size or thickness (regular paper, thick paper, and the like), and film sheet such as an overhead projector (hereinafter, OHP) may also be used. When the paper feed cassette  41  is provided in plurality, recording sheets differing in terms of size, thickness, or quality may be contained in the respective paper feed cassettes. 
     The timing roller  45  transports the recording sheet to the secondary transfer position  46  in accordance with timing at which each unfixed image having undergone the primary transfer on the intermediate transfer belt  11  is transported to the secondary transfer position  46  to achieve overlay transfer at the same position on the intermediate transfer belt  11 . Next, at the secondary transfer position  46 , the unfixed images on the intermediate transfer belt  11  undergo the static transfer onto the recording sheet all at once, performed by the secondary transfer roller  47 . 
     The various rollers, such as the feed roller  42 , the timing roller  45 , the exit roller  71 , and the transport rollers  73 ,  74 ,  76  and  77  have a transport motor (not diagrammed) serving as a drive power source, and are driven to rotate through a power transmission mechanism (not diagrammed) including toothed gears, belts, and the like. The transport motor may be, for example, a stepping motor capable of high-precision rotation speed control. 
     The fixing device  5  includes a heat roller  51  (here, for example, the heat roller is heated by a heater) and a pressure roller  52  pressing the heat roller  51 . A fixing nip is formed between the rollers, and the thermal fixing of the unfixed image occurs at the fixing nip. The position at which the fixing nip is formed is hereinafter termed a fixing position, and is indicated by reference sign  53  in  FIG. 1 . 
     Also, a heat roller temperature sensor  500  is provided in the vicinity of the heat roller  51 , and measures a surface temperature of the heat roller  51 . The control unit  60  controls the surface temperature of the heat roller  51  by controlling the power supplied to the heat roller  51  (or to the heater of the heat roller  51 ). 
     Although not illustrated, the fixing device  5  is provided with a frame supporting both longitudinal ends of each of the heat roller  51  and the pressure roller  52 , and covering these components. The frame is provided with a gap, as required, in the vicinity of the entrance and exit for the recording sheet and in the vicinity of where the frame supports the longitudinal ends of the heat roller  51  and the pressure roller  52 . 
     [2] Control Unit Configuration 
       FIG. 2  depicts the configuration of the control unit  60  and the relationship between the control unit  60  and the main components subject to control. The control unit  60  is a computer that, as depicted, includes a central processing unit (hereinafter, CPU)  600 , a communication interface unit  601 , read-only memory (hereinafter, ROM)  602 , random access memory (hereinafter, RAM)  603 , an image data storage unit  604 , a sheet position detection unit  605 , a sheet position storage unit  606 , a parameter storage unit  607 , and an image region determination unit  608 . 
     The communication interface unit  601  is an interface for connecting to a local area network (hereinafter, LAN) such as a LAN card, a LAN port, or the like. The ROM  602  stores programs for controlling the image processing unit  3 , the feed unit  4 , the fixing device  5 , a transfer voltage output unit  6 , an operation panel  7 , an image acquisition unit  8 , the heat roller temperature sensor  500 , and the sheet passing sensor  81 , as well as programs for executing a later-described fixing temperature control process and both-side printing transfer voltage control process. 
     The RAM  603  is used as a work area by the CPU  600  during program execution. 
     The image data storage unit  604  stores image data for printing, input through the communication interface unit  601  and the image acquisition unit  8 . 
     The sheet position detection unit  605  counts a quantity of drive pulses of the transport motor after a leading edge of the recording sheet has passed the sheet passing sensor  81 , and thereby calculates a transport distance of the recording sheet relative to the sheet passing sensor  81  and detects current positions of the leading edge and a trailing edge of the recording sheet along the transport channel  44 . The quantity of drive pulses is, for example, detectable by counting the drive pulses supplied to the transport motor by the control unit  60 . 
     The sheet position storage unit  606  stores a quantity of drive pulses corresponding to each of the secondary transfer position  46  and the fixing position  53 , and a quantity of drive pulses pertaining to the size of the recording sheet. 
     Specifically, the sheet position storage unit  606  stores each of the quantity of drive pulses corresponding to the transport distance between a detection position of the sheet passing sensor  81  and the secondary transfer position  46 , the quantity of drive pulses corresponding to the transport distance between the detection position of the sheet passing sensor  81  and the fixing position  53 , and the quantity of drive pulses required to perform transport over a distance corresponding to the size of the recording sheet in a sheet passing direction. 
     The sheet position detection unit  605  compares the counted quantity of drive pulses to the respective quantities of drive pulses stored in the sheet position storage unit  606  corresponding to the secondary transfer position  46  and to the fixing position  53 , and detects the leading edge of the recording sheet as reaching the secondary transfer position  46  and the fixing position  53  when the counted quantity of drive pulses reaches the respective quantity of drive pulses used for comparison. The sheet position detection unit  605  also compares the respective quantity of drive pulses counted once the leading edge reaches the secondary transfer position  46  and the fixing position  53  to the quantity of drive pulses required to perform transport of the distance corresponding to the size of the recording sheet in the sheet passing direction, and detects the trailing edge of the recording sheet as reaching the secondary transfer position  46  and the fixing position  53  when the counted quantity of drive pulses reaches the respective quantity of drive pulses used for comparison. 
     The parameter storage unit  607  stores an economy temperature, an upper limit temperature, a lower limit transfer voltage, an upper limit transfer voltage, and a transfer voltage calculation formula. Here, the economy temperature is a temperature at which thermal fixing of a text image is possible, and is a lower limit thermal fixing temperature at which the surface temperature of the heat roller  51  is maintained during thermal fixing by the image forming device  1 . Also, the upper limit temperature is a temperature at which thermal fixing of a color image is possible, and is an upper limit thermal fixing temperature at which the surface temperature of the heat roller  51  is maintained during thermal fixing of the image forming device  1 . 
     The economy temperature and the upper limit temperature are determined in advance by the manufacturer of the image forming device  1  through testing or the like. Here, for example, the economy temperature is 150° C. and the upper limit temperature is 165° C. 
     The lower limit transfer voltage is the transfer voltage to be applied during both-side printing, when thermal fixing has been performed on one side at the economy temperature, and the static transfer of the unfixed image is performed on the other side. Here, for example, the lower limit transfer voltage is 500 V. 
     The upper limit transfer voltage is the transfer voltage applied during both-side printing when thermal fixing has been performed on one side at the upper limit temperature, and the static transfer of the unfixed image is performed on the other side. Here, for example, the upper limit transfer voltage is 800 V. The lower limit transfer voltage and the upper limit transfer voltage are determined in advance by the manufacturer of the image forming device  1  through testing or the like. 
     The transfer voltage calculation formula is used in the later-described both-side printing transfer voltage control process to calculate the transfer voltage (V) applied when the other side of the recording sheet passes the secondary transfer position  46 . Specifically, the following formula is stored.
 
 V =(( T−T ec)/( T max− T ec))×( V max− V min)+ V min
 
     In the above-described formula, Tec represents the economy temperature, Tmax represents the upper limit temperature, Vmax represents the upper limit transfer voltage, and Vmin represents the lower limit transfer voltage. Also, T represents a fixing position temperature (index temperature of the sheet temperature) at a sampling sheet position. Fixing position temperatures are acquired during the later-described both-side printing transfer voltage control process. Here, sampling sheet positions are positions of the recording sheet in the sheet passing direction at which the fixing position temperatures are acquired upon passing the fixing position  53 , in a later-described sheet passing direction fixing temperature distribution sampling process. 
     Each fixing position temperature is an index value representing a relative water content at the corresponding sampling sheet position (a higher fixing position temperature being indexed to lower water content). Here, surface temperatures of the heat roller  51  detected by the heat roller temperature sensor  500  when the recording sheet passes the fixing position  53  is used as the fixing position temperatures. 
       FIG. 3  describes the relationship between the sampling sheet positions and the fixing position temperatures. Here, the dashed rectangles having the reference sign P each represent the recording sheet, and the solid-line arrows represent the sheet passing direction of the recording sheet (i.e., the transport direction toward the fixing position  53  (marked by the black triangle)).  FIG. 3  indicates how the recording sheet P gradually travels in the sheet passing direction as time elapses. In specific,  FIG. 3  indicates a course along which the recording sheet P travels during the period of time from when the leading edge of the recording sheet P passes through the fixing position  53  until when the trailing edge of the recording sheet P passes through the fixing position  53 . The amount of time having elapsed (elapsed time) from when the leading edge of the recording sheet P passes through the fixing position  53  increases from the bottom to the top of the image. 
     Dashed arrows S 0  through S 5  indicate the respective sampling sheet positions on the recording sheet P at which the fixing position temperatures (i.e., fixing temperatures T 0  through T 5 ) are acquired. Here, the fixing position temperatures are acquired at a predetermined time interval t between the period from when the leading edge of the recording sheet P passes through the fixing position  53  (taken as time  0 ) until when the trailing edge of the recording sheet P passes through the fixing position  53 . 
       FIG. 4  is a table indicating the relationship between the sampling sheet positions illustrated in  FIG. 3 , the elapsed time since the passing of the leading edge of the recording sheet P through the fixing position  53  in  FIG. 3 , and the fixing position temperatures. 
     In the later-described sheet passing direction fixing temperature distribution sampling process, as described above, the fixing position temperatures are acquired at a predetermined interval while the recording sheet passes through the fixing position  53 , and the fixing position temperatures so acquired are used as the index value of water content at each position in the sheet passing direction of the recording sheet (i.e., each sampling sheet position). 
     Returning to  FIG. 2 , in the above-described formula, the transfer voltage (V) is the lower limit transfer voltage (Vmin) when T is the economy temperature (Tec), the transfer voltage (V) is the upper limit transfer voltage when T is the upper limit temperature (Tmax), and the transfer voltage (V) increases within a range not exceeding the upper limit transfer voltage (Vmax) for increasing values of T (i.e., for lower water content) when T is between the economy temperature and the upper limit temperature. 
     Accordingly, the transfer voltage is determined for each sampling sheet position using the above-described formula, which enables the transfer voltage to be determined so as to cancel out the effect of changes in electrical resistance caused by variations in water content. 
     For example, when the water content is low and the electrical resistance is high at a given sampling sheet position of the recording sheet, then the transfer voltage flows through the given sampling sheet position with difficulty. As such, in this case, the transfer voltage (V) is determined such that the absolute value of the transfer voltage applied at the given sampling sheet position is relatively large. This enables the effect of the above-described variations to be canceled out. 
     The image region determination unit  608  determines, in accordance with image data for each page, whether an image represented by image data is a color image or a monochrome image, and whether or not an image represented by the image data includes a photographic image. 
     The determination of whether or not an image is a color image is made, for example, by counting a quantity of pixels to which each color of toner Y, M, C, and K is applied (hereinafter termed toner-applied pixels) within the image data and determining whether or not the quantity of pixels is zero for three of the colors. That is, when the quantity of toner-applied pixels is zero for three colors, the image is found to be a monochrome image, and otherwise the image is found to be a color image. 
     Also, the determination of whether or not the image data includes a photographic image is made, for example, by acquiring, in each of a main scan direction and a sub-scan direction, a distribution of a total pixel quantity within the image data for printing one page stored in the image data storage unit  604 , and making the determination by detecting regularity in the distribution. 
     When regularities are found in the entirety of the image data for one page, or when the image data is found to include portions of regularity and blank portions, then the image in that page is found not to include a photographic image. Conversely, when regularity is observed only in a part of the image or when no regularity is observed in the entirety of the image, then the image is found to include a photographic image. 
     For a text image, the total quantity of toner-applied pixels is zero in the spaces between rows and columns in which the text is arranged. Thus, regularity is observed wherever these portions having zero toner-applied pixels repeat with regular spacing. Detecting such regularity enables the determination to be made. (See also Japanese Patent Application Publication No. 2007-259466, paragraphs 0058 through 0060 and FIGS. 6 and 7.) 
     Also, when image data written in page description language (hereinafter, PDL) is acquired from a terminal device, the determination of whether or not each page of image data includes a photographic image may be made by analysis of the PDL. 
     The CPU  600  controls the image processing unit  3 , the feed unit  4 , the fixing device  5 , a transfer voltage output unit  6 , the operation panel  7 , the image acquisition unit  8 , the heat roller temperature sensor  500 , and the sheet passing sensor  81 , by executing the programs stored in the ROM  602 , and executes the later-described fixing temperature control process and both-side printing transfer voltage control process. 
     The transfer voltage output unit  6  applies the transfer voltage to the secondary transfer roller  47 . The transfer voltage is applied in accordance with control by the control unit  60 . The operation panel  7  includes a liquid crystal display, a touch panel superposed on the liquid crystal display or operation buttons for various input, and the like. The operation panel  7  receives input of various instructions from a user via the touch panel, the operation buttons, or the like. 
     The image acquisition unit  8  includes an image input device such as a scanner, and forms image data by acquiring text, shapes, pictures, and similar image information from a recording sheet of paper or the like. 
     [3] Fixing Temperature Control Process 
       FIG. 5  is a flowchart indicating operations of the fixing temperature control process performed by the control unit  60 . The control unit  60  acquires a print job indicating image data and printing conditions through the communication interface unit  601  or through the operation panel  7  and the image acquisition unit  8  (step  501 ), executes image processing on the image data for each page of the acquired print job (step S 502 ), acquires the image data for printing in the bitmap format as image information (step  503 ), and then executes each of a later-described target temperature setting process and inter-page fixing temperature adjustment process (step S 504 , step S 505 ). 
       FIG. 6  is a flowchart indicating the operations of the target temperature setting process. The control unit  60  determines, based on image information having been acquired corresponding to a given page of the acquired print job, whether or not the image indicated by the image information is a color image (step S 601 ). 
     When the image indicated by the image information is a color image (YES in step S 601 ), the control unit  60  sets the target temperature at which the surface temperature of the heat roller  51  is to be maintained during thermal fixing of the page to the upper limit temperature (step S 603 ). 
     When the image indicated by the image information for the page is a monochrome image (NO in step S 601 ), the control unit  60  further determines whether or not the image indicated by the image information includes a photographic image (step S 602 ). 
     When the result of step S 602  is negative (NO in step S 602 ), the control unit  60  sets the target temperature for the page to the economy temperature (step S 604 ). 
     When the result of step S 602  is affirmative (YES in step S 602 ), the control unit  60  transitions to step S 603 . 
       FIG. 7  is a flowchart indicating the operations of the inter-page fixing temperature adjustment process. The control unit  60  performs the printing process for each page of the acquired print job, and upon beginning the print process for a given page, determines whether or not the target temperature set for the page is the economy temperature (step S 701 ). 
     When the result of step S 701  is affirmative (YES in step S 701 ), the control unit  60  further determines whether or not the target temperature for a page following the current page is set to the upper limit temperature (step S 702 ). 
     When the result of step S 702  is affirmative (YES in step S 702 ), the control unit  60  controls electric power supplied to the heat roller  51  so that, after beginning thermal fixing of the current page at the economy temperature, the surface temperature of the heat roller  51  reaches the upper limit temperature by the beginning of thermal fixing for the next page, thus causing the surface temperature to increase during the thermal fixing of the page (step S 703 ). 
     When the result of step S 702  is negative (NO in step S 702 ), the control unit  60  controls electric power supplied to the heat roller  51  so that the surface temperature of the heat roller  51  is maintained at the economy temperature during the thermal fixing of the page (step S 704 ). 
     Also, when the target temperature for a previous page preceding the current page is the upper limit temperature, the control unit  60  stops the electric power supply to the heat roller  51  upon beginning the thermal fixing of the current page until the surface temperature reaches the economy temperature, thus causing the surface temperature to decrease to the economy temperature. 
     Also, when the result of step S 701  is negative (NO in step S 701 ), the control unit  60  further determines whether or not the target temperature for the page following the current page is set to the economy temperature (step S 705 ). When the result of step S 705  is negative (NO in step S 705 ), the control unit  60  then controls electric power supplied to the heat roller  51  so that the surface temperature of the heat roller  51  is maintained at the upper limit temperature during the thermal fixing of the page (step S 706 ). 
     Conversely, when the result of step S 705  is affirmative (YES in step S 705 ), the control unit  60  controls the electric power supplied to the heat roller  51  to maintain the surface temperature of the heat roller  51  at the upper limit temperature, similarly to the process of step S 706 , and once the period for thermal fixing the current page ends, stops the electric power supply to the heat roller  51  and causes the surface temperature to decrease (step S 707 ). 
     The control unit  60  then transitions to step S 701  when the current page is not a final page (NO in step S 708 ). 
     [4] Both-Side Printing Transfer Voltage Control Process 
       FIG. 8  is a flowchart indicating the operations of the both-side printing transfer voltage control process performed by the control unit  60 . When the acquired print job indicates both-side printing as a print condition, the control unit  60  causes the image processing unit  3  to begin image formation for a page on the front side (step S 801 ), and once an image for the page on the front side is formed, causes the feed unit  4  to begin feeding a recording sheet (step S 802 ), applies a predetermined transfer voltage to the secondary transfer roller  47  through the transfer voltage output unit  6 , and statically transfer the unfixed image formed by the image processing unit  3  onto the front side of the recording sheet at the secondary transfer position  46  (step S 803 ). 
     Then, once the leading edge of the recording sheet reaches the fixing position  53  after the static transfer of the unfixed image onto the front side (YES in step S 804 ), the control unit  60  executes the later-described sheet passing direction fixing temperature distribution sampling process (step S 805 ), substitutes the fixing position temperatures (T) at the sampling sheet positions acquired in step S 805  into the transfer voltage formula stored in the parameter storage unit  607 , calculates applied transfer voltages (V) to be applied to the back side of the recording sheet when passing the secondary transfer position  46  to determine the applied transfer voltages (V) for the sampling sheet positions, and stores, in the RAM  603 , an output transfer voltage table listing the sampling sheet positions, the elapsed time and fixing position temperature pertaining to each of the sampling sheet positions, and the determined applied transfer voltages (V) in correspondence (step S 806 ). 
       FIG. 9  is a specific example of the output transfer voltage table. As indicated in  FIG. 9 , for each of six sampling sheet positions (S 0 , S 1 , S 2 , S 3 , S 4 , S 5 ), an elapsed time ( 0 , t,  2   t ,  3   t ,  4   t ,  5   t ), a fixing position temperature (T 0 , T 1 , T 2 , T 3 , T 4 , T 5 ), and an applied transfer voltage (V 0 , V 1 , V 2 , V 3 , V 4 , V 5 ) are listed in correspondence. 
     For example, (i) when the target fixing temperature of the page on the front side of the recording sheet is the economy temperature, the target fixing temperature of the page on the back side of the recording sheet, which is the next page, is the upper limit temperature, and thus control for increasing the temperature in step S 703  is performed in the inter-page fixing temperature adjustment process in  FIG. 7 , and (ii) T 0 =150° C., T 1 =153° C., T 2 =154° C., T 3 =157° C., T 4 =158° C., T 5 =160° C., and thus the applied transfer voltages (V) calculated using the transfer voltage formula are V 0 =500 V, V 1 =560 V, V 2 =580 V, V 3 =640 V, V 4 =660 V, and V 5 =700 V, the relationship between the sampling sheet positions, the fixing position temperatures, and the applied transfer voltages in the output transfer voltage table can be illustrated as the graph in  FIG. 10 . 
     In  FIG. 10 , reference signs S 0  through S 5  indicate the sampling sheet positions, reference signs T 0  through T 5  indicate the fixing position temperature, and reference signs V 0  through V 5  indicate the applied transfer voltages calculated using the transfer voltage formula. Also, the dashed arrow indicates the water content, increasing in the direction of the arrow. 
     As indicated, the applied transfer voltage at a given sampling sheet position is set to have a larger absolute value for a higher fixing position temperature at the sampling sheet position and thus lower water content at the sampling sheet position. 
     Returning to  FIG. 8 , the control unit  60  then causes the image processing unit  3  to begin forming an image for the page on the back side (step S 807 ) and, once the image for the page has been formed, causes the feed unit  4  to begin feeding the recording sheet. Then, once the leading edge of the recording sheet that is fed is detected at the secondary transfer position  46  (YES in step S 808 ), the control unit  60  executes the later described transfer voltage control process (step S 809 ) and causes the fixing device  5  to thermally fix the unfixed image having been statically transferred onto the back side of the recording sheet (step S 810 ). 
     Note that a non-diagrammed sheet passing sensor is provided at a predetermined position along the reverse transport channel  75 . Once this sheet passing sensor detects the passing of the recording sheet and the sheet passing sensor  81  detects passing of the recording sheet after an interval of time corresponding to a transport distance from the position of the non-diagrammed sheet passing sensor to the detection position of sheet passing sensor  81  has elapsed, the control unit  60  performs the above-described processing of step S 809 . 
     The processing of steps  801  through S 810  is then repeated until the print job is complete (YES in step S 811 ). 
       FIG. 11  is a flowchart indicating the operations of the sheet passing direction fixing temperature distribution sampling process. The control unit  60  begins a time measurement and acquires, from the heat roller temperature sensor  500 , the fixing position temperature (T 0 ) at the initial time t 0  (elapsed time zero seconds) of the time measurement. The values of t 0  and T 0  are associated with an identifier (S 0 ) for the sampling sheet position at which the fixing position temperature (T 0 ) is acquired and stored in the RAM  603  (step S 1101 ). 
     Then, once a predetermined interval (here, 20 ms, for example) has elapsed since the preceding acquisition of the fixing position temperature (YES in step S 1102 ), the next fixing position temperature (T) is acquired. The elapsed time since t 0  (t) and T are associated with an identifier (S) indicating the sampling sheet position at which the fixing position temperature (T) is acquired, and are stored in the RAM  603  (step S 1103 ). 
     Next, the control unit  60  determines whether or not the trailing edge of the recording sheet has reached the fixing position  53  (step S 1104 ). Steps S 1102  and S 1103  are repeated until the trailing edge reaches the fixing position  53  (YES in step S 1104 ). 
       FIG. 12  is a flowchart indicating the operations of the transfer voltage control process. The control unit  60  begins the time measurement and references the output transfer voltage table, then causes the transfer voltage output unit  6  to output the applied transfer voltage V corresponding to the sampling sheet position for elapsed time zero to the secondary transfer roller  47  (step S 1201 ). 
     Then, at the time when the next sampling sheet position reaches the secondary transfer position  46  (YES in step S 1202 ), the control unit  60  causes the transfer voltage output unit  6  to output the applied transfer voltage (V) corresponding to the next sampling sheet position to the secondary transfer roller  47  (step S 1203 ). 
     Here, fixing position temperatures are acquired at a predetermined time interval (see steps S 1102  and S 1103  in  FIG. 11 ). As such, the time at which the next sampling sheet position reaches the secondary transfer position  46  in step S 1202  occurs each time the predetermined interval elapses since the arrival of the previous sampling sheet position at the secondary transfer position  46  (e.g., when the predetermined interval elapses since the sampling sheet position for elapsed time zero). Here, the recording sheet is transported to the fixing position  53  and to the secondary transfer position  46  at equal transport speeds. 
     Next, the control unit  60  determines whether or not the trailing edge of the recording sheet has reached the secondary transfer position  46  (step S 1204 ). Steps S 1202  and S 1203  are repeated until the trailing edge reaches the secondary transfer position  46  (YES in step S 1204 ). 
       FIG. 13  schematically illustrates how the applied transfer voltage that is output is switched each time one of the sampling sheet positions reaches the secondary transfer position  46 . Here, the dashed rectangles having the reference sign P each represent the recording sheet, and the solid line arrows represent the sheet passing direction of the recording sheet (i.e., the transport direction toward the secondary transfer position  46  (marked by the black triangle).  FIG. 13  indicates how the recording sheet P gradually travels in the sheet passing direction as time elapses. In specific,  FIG. 13  indicates a course along which the recording sheet P travels during the period of time from when the leading edge of the recording sheet P passes through the secondary transfer position  46  until when the trailing edge of the recording sheet P passes through the secondary transfer position  46 . The amount of time having elapsed (elapsed time) from when the leading edge of the recording sheet P passes through the secondary transfer position  46  increases from the bottom to the top of the image. 
     In  FIG. 13 , the positions indicated by dashed arrows S 0  through S 5  represent the sampling sheet positions, and the white arrows indicate the applied transfer voltages at the sampling sheet positions. Also, as indicated in  FIG. 13 , the applied transfer voltage V 0 , V 1 , V 2 , V 3 , V 4 , and V 5  is switched at each of the six sampling sheet positions S 0 , S 1 , S 2 , S 3 , S 4 , and S 5 . 
     Accordingly, in the present Embodiment, during both-side printing, the fixing position temperature at each sampling sheet position on the front side is acquired as an index of water content during thermal fixing of the front side. Then, when statically transferring an unfixed image onto the back side at the secondary transfer position  46 , the transfer voltage applied at the secondary transfer position  46  with respect to each sampling sheet position is set so that the absolute value of the applied transfer voltage increases as the water content indicated by the fixing position temperature at the sampling sheet position decreases. As such, despite variations in water content in the recording sheet during both-side printing, the effect of fluctuations in electrical resistance caused by these variations is canceled out, thus enabling the static transfer of the unfixed image on the back side to be performed without unevenness. As a result, degradation in image quality on the back side is prevented. 
     (Modifications) 
     The above description of the disclosure has been provided in terms of the Embodiment. However, no limitation is intended to the above-described Embodiment. The following modifications are also applicable. 
     (1) In the Embodiment, fixing position temperatures at a plurality of positions along the sheet passing direction are acquired as index values indexing the water content. However, another method may also be used to acquire the index values. For example, an optical water sensor may be used to measure the water content in the recording sheet at a plurality of positions in the sheet passing direction after thermal fixing on the front side. Alternatively, an average temperature increase rate per unit time may be calculated from a difference in target temperatures for pages, and the calculated average temperature increase rate may be used to calculate the temperatures of a plurality of positions in the sheet passing direction after the thermal fixing on the front side, and the temperatures so calculated may be used as the index value.
 
(2) In the Embodiment, the both-side printing transfer voltage control process is performed during both-side printing to prevent transfer unevenness caused by variations in water content of the recording sheet. However, recording sheet curling may occur during one-side printing due to the variations in water content, depending upon the level of variation. As such, a process of correcting such curling in accordance with the variations in water content may also be performed.
 
     Specifically, as indicated in  FIG. 14 , the image forming device  1  may include a post-processing device  9  equipped with a de-curling mechanism, and the control unit  60  may cause the post-processing device  9  to execute a later-described one-side printing curl control process. 
     The post-processing device  9  includes a post-processing control unit  90 , a channel switching member  91 , and a de-curling mechanism  92 . In the present modification, the recording sheet having undergone thermal fixing of the unfixed image in the fixing device  5  is transported into the post-processing device  9 , passes through one of a default transport channel  93  that does not go through the de-curling mechanism  92  and a de-curling transport channel  94  that goes through the de-curling mechanism  92 , and then exits the post-processing device  9 . 
     The control unit  60  is able to communicate with the post-processing control unit  90  and controls the operations of the post-processing device  9  via the post-processing control unit  90 . 
     The post-processing control unit  90  includes a CPU, ROM, RAM, and the like, and controls the channel switching member  91  and the de-curling mechanism  92 , and performs overall control of the post-processing device  9 , in response to an instruction from the control unit  60 . The channel switching member  91  is a member switching the transport channel into which the recording sheet is transported, within the post-processing device  9 . 
     As illustrated in  FIG. 15 , the de-curling mechanism  92  includes a plurality of curl correction units  901  through  905  differing in terms of curl correction direction and correction power. Each of the curl correction units  901  through  905  includes three rollers and an endless belt. The endless belts (belts B 1  through B 5 ) are respectively extended across two of the rollers while the third roller is in contact with an external circumferential surface of the endless belt and presses the endless belt inward, thus forming nips N 1  through N 5  between each endless belt and the third roller. The recording sheet P is transported in the direction indicated by arrow D along the transport channel  910  indicated by the dashed line, and sequentially passes through nips N 1  through N 5 . The curling is thus corrected in each of the nips N 1  through N 5 . 
     In  FIG. 15 , reference signs  901 A,  901 B,  902 A,  902 B,  903 A,  903 B,  904 A,  904 B,  905 A, and  905 B indicate the suspension rollers on which the endless belts B 1  through B 5  are suspended, and reference signs  901 C,  902 C,  903 C,  904 C, and  905 C indicate the rollers in external contact with the respective endless belts B 1  through B 5 . 
     Within the de-curling mechanism  92 , the arrangement and size of the rollers are adjusted so that neighboring curl correction units in the transport direction of the recording sheet P apply the curl correction in opposing directions. The curl correction force applied between curl correction units in the same correction direction (i.e., between curl correction units  901 ,  903 , and  905 , and between curl correction units  902  and  904 ) decreases gradually from an upstream side to a downstream side of the transport direction (i.e., the pressure by the roller on the endless belt is smaller and the outer radius of the roller is larger). 
       FIG. 16  is a flowchart indicating the operations of the one-side printing curl control process performed by the control unit  60 . The processing of steps S 1601  through S 1604  is identical to the processing of steps S 801  through S 804  from  FIG. 8 , and explanations thereof are thus omitted. 
     When the result of step S 1604  is affirmative (YES in step S 1604 ), the control unit  60  executes a later-described sheet passing direction fixing temperature change detection process (step S 1605 ), and determines whether or not a difference d between a maximum value (Tmax) and a minimum value (Tmin) of the fixing position temperatures in the sheet passing direction during thermal fixing as calculated during step S 1605  exceeds a threshold (step S 1606 ). 
     Here, the threshold is a value corresponding to a tolerable upper limit at which curling does not occur, and is determined through testing or the like and set in advance by the manufacturer of the image forming device. 
     When the result of step S 1606  is affirmative (YES in step S 1606 ), the control unit  60  controls the channel switching member  91  of the post-processing device  9  through the post-processing control unit  90  to switch the transport channel to the de-curling transport channel  94  and transport the recording sheet, having been transported into the post-processing device  9  after thermal fixing, to the de-curling mechanism  92  where the de-curling mechanism  92  applies curl correction to the recording sheet (step S 1607 ). 
     When the result of step S 1606  is negative (NO in step S 1606 ), the control unit  60  controls the channel switching member  91  of the post-processing device  9  through the post-processing control unit  90  to switch the transport channel to the default transport channel  93  and transport the recording sheet, having been transported into the post-processing device  9  after thermal fixing, without passing through the de-curling mechanism  92  and without curl correction being applied to the recording sheet, directly outside the post-processing device  9  (step S 1608 ). 
     Next, the control unit  60  determines whether or not the acquired print job is complete (step S 1609 ). Steps S 1601  through S 1608  are repeated until the print job is complete (YES in step S 1609 ). 
       FIG. 17  is a flowchart indicating the operations of the sheet passing direction fixing temperature change detection process. The control unit  60  begins the time measurement and acquires, from the heat roller temperature sensor  500 , the fixing position temperature (T 0 ) at the initial time t 0  (elapsed time 0 seconds) at the start of the time measurement (step S 1701 ). 
     The control unit  60  then takes T 0  as the value of the variable Tmax indicating the maximum value of the fixing position temperatures and the variable Tmin indicating the minimum value of the fixing position temperatures (step S 1702 ). Once a predetermined interval (here, 20 ms, for example) has elapsed since the acquisition of a previous fixing position temperature (YES in step S 1703 ), the control unit  60  acquires the next fixing position temperature (T) (step S 1704 ). 
     Next, the control unit  60  compares the values of T and Tmax. When T is greater than Tmax (YES in step S 1705 ), then T is set to the value of Tmax (step S 1706 ). When T is not greater than Tmax (NO in step S 1705 ), the control unit  60  further compares the values of T and Tmin. When T is less than Tmin (YES in step S 1707 ), then T is set to the value of Tmin (step S 1708 ). 
     Furthermore, the control unit  60  determines whether or not the leading edge of the recording sheet has reached a predetermined position, in accordance with detection results from the sheet position detection unit  605  (step S 1709 ). 
     Here, the predetermined position is determined by the manufacturer of the image forming device  1  in accordance with the length of the transport channel  44 . For example, when the transport channel  44  from the fixing position  53  to the exit to the post-processing device  9  is long, and the leading edge of the recording sheet does not reach the exit by the time the trailing edge of the recording sheet passes the fixing position  53 , then the predetermined position is set to a position along the transport channel  44  passed by the leading edge of the recording sheet when the trailing edge reaches the fixing position  53 . 
     Conversely, when the transport channel  44  is short and the leading edge of the recording sheet reaches the exit before the trailing edge of the recording sheet reaches the fixing position  53 , then, for example, the position of the exit serves as the predetermined position. 
     In the above-described circumstances, a drive pulse quantity corresponding to the transport distance from the detection position of the sheet passing sensor  81  to the predetermined position is stored in the sheet position storage unit  606 . The control unit  60  compares the drive pulse quantity counted by the sheet position detection unit  605  and the drive pulse quantity corresponding to the distance to the predetermined position, and detects the leading edge of the recording sheet as having reached the predetermined position when the drive pulse quantities being compared are equalized. 
     Accordingly, step S 1606  is performed before the leading edge of the recording sheet is transported into the post-processing device  9 , which enables the channel switching member  91  to perform the transport channel switching in time. 
     Also, when the result of step S 1709  is affirmative (YES in step S 1709 ), the control unit  60  calculates the difference d between the maximum value (Tmax) and the minimum value (Tmin) of the fixing position temperatures in the sheet passing direction during thermal fixing (step S 1710 ). 
     When the result of step S 1709  is negative (NO in step S 1709 ), the control unit  60  transitions to step S 1703 . Also, when the result of step S 1707  is negative (NO in step S 1707 ), the control unit  60  transitions to step S 1709 . 
     As such, in the present modification, during one-side printing, the fixing position temperatures in the sheet passing direction during thermal fixing are acquired as the index values of water content. When the variation in water content in the sheet passing direction exceeds a threshold, the recording sheet may experience curling. As such, the de-curling mechanism controls curling by applying correction, thus preventing curling caused by the variation in water content during thermal fixing. 
     Also, within the de-curling mechanism  92 , the arrangement of the external rollers may be adjustable such that the amount of pressing by the external rollers with respect to the endless belts changes in accordance with the magnitude of d (the greater the value of d, the greater the amount of pressing). 
     An actuator or the like may be used as a displacement mechanism for the rollers. Such a displacement mechanism is controlled by the control unit  60  via the post-processing control unit  90  such that the amount of pressing is controlled in accordance with the magnitude of d, and the curl correction is greater for greater values of d. As a result, the curl correction force is adjusted in accordance with the degree of curling, enabling the curl correction to be optimized as neither too weak nor too strong. 
     (3) Also, in modification (2), when the grammage of the recording sheet is low and mechanical correction is unable to fully correct the curling, then control may be performed to subject the recording sheet to humidification by a humidifier and then perform curl correction in the de-curling mechanism. 
     Specifically, as indicated in  FIG. 18 , the image forming device  1  may include a post-processing device  9 A equipped with a humidifier  97  and a de-curling mechanism  92 , and the control unit  60  may cause the post-processing device  9 A to execute a later-described one-side printing curl control process. 
     The post-processing device  9 A of  FIG. 18  is configured similarly to the post-processing device  9  of  FIG. 14 . As such, the same reference signs are used and explanations of similar components are omitted. The following mainly describes the points of difference relative to the post-processing device  9  of  FIG. 14 . 
     The post-processing device  9 A control unit includes a post-processing control unit  90 , channel switching members  91  and  96 , a de-curling mechanism  92 , and a humidifier  97 . Also, in the post-processing device  9 A, the recording sheet that has been transported to a de-curling transport channel  94  after thermal fixing is guided to a transport channel by the channel switching member  96 , the transport channel being one of a detour transport channel  98  heading to the humidifier  97  and a non-detour transport channel  99  heading directly to the de-curling mechanism  92 . 
       FIG. 19  illustrates a specific example of the humidifier  97 . As illustrated, the humidifier  97  includes a pair of humidity rollers  971  applying moisture to the recording sheet, which is indicated by reference sign P and transported in the direction indicated by the arrow, a water supply roller  972  in contact with the pair of humidity rollers  971  and supplying water thereto, a control member  973  controlling the water supplied from the water supply roller  972  to the humidity rollers  971  by pressing into the outer circumferential surface of the water supply roller  972 , and a water storage container  974 . The water storage container  974  stores water  975 . 
     Each roller is, for example, made from a shaft  971 A,  971 B, and  972 A of metal, cured resin, or the like, and a respective porous layer  971 C,  971 D, and  972 B made of porous urethane resin or similar formed around the circumference of each shaft  971 A,  971 B, and  972 A. 
     The humidity rollers  971  and the water supply roller  972  are driven to rotate in the direction of the arrow by a non-diagrammed drive motor. The drive motor is controlled by the post-processing control unit  90 . 
     Here, the humidifier  97  is not limited to the above-described configuration provided that humidity is evenly applied to the recording sheet. For example, the recording sheet may be humidified by spraying with water vapor. 
       FIG. 20  is a flowchart indicating a modification of the operations of the one-side printing curl control process indicated in  FIG. 16 . In  FIG. 20 , steps S 1601  to S 1605  representing processing identical to that of  FIG. 16  uses identical step reference signs, and explanations thereof are omitted. The following mainly describes points of difference. 
     When the result of step S 1606  is affirmative (YES in step S 1606 ), the control unit  60  determines whether or not the grammage of the recording sheet is equal to or less than a grammage threshold (step S 2001 ). 
     Then, when the result of step S 2001  is affirmative (YES in step S 2001 ), the control unit  60  controls the transport channel switching members  91  and  96  of the post-processing device  9 A through the post-processing control unit  90  to switch the transport channel to the de-curling transport channel  94  and the detour transport channel  98 , and thus transport the recording sheet, having been transported into the post-processing device  9 A after thermal fixing, to the de-curling mechanism  92  via the humidifier  97 , where the humidifier  97  humidifies the recording sheet and the de-curling mechanism then applies curl correction to the recording sheet (step S 2002 ). 
     When the result of step S 2001  is negative (NO in step S 2001 ), the control unit  60  controls the transport channel switching members  91  and  96  of the post-processing device  9 A through the post-processing control unit  90  to switch the transport channel to the de-curling transport channel  94  and the non-detour transport channel  99 , thus transporting the recording sheet having been transported into the post-processing device  9 A after thermal fixing directly to the de-curling mechanism  92  where the de-curling mechanism  92  applies curl correction to the recording sheet (step S 2003 ). 
     As such, according to the present modification, when the grammage of the recording sheet is equal to or less than the grammage threshold, the humidifier  97  applies humidity to the recording sheet such that mechanical correction of the curl is applied after fiber resilience in the recording sheet has been lowered. Thus, insufficient curl correction is prevented from occurring, even in a thin recording sheet with low grammage. 
     (4) In the Embodiment, the heat roller  51  is assumed to have an even surface temperature, and the beginning of fixing position temperature acquisition coincides with the arrival of the leading edge of the recording sheet at the fixing position  53 . However, given the offset in terms of distance between the fixing position  53  and the detection position at the outer circumferential surface of the heat roller  51  where the heat roller temperature sensor  500  performs detection, in order to reduce a temperature error caused by the offset, the beginning of the fixing position temperature acquisition may precede the arrival of the leading edge of the recording sheet at the fixing position  53  by time Δt required for the heat roller  51  to rotate by an amount corresponding to the distance. 
     Specifically, during the both-side printing transfer voltage control process of  FIG. 8 , the timing at which the sheet passing direction fixing temperature distribution sampling process begins in step S 805  may be earlier than the arrival of the leading edge of the recording sheet at the fixing position  53  by Δt. 
     That is, surface temperatures of the heat roller  51  may be acquired at time points earlier by Δt than the time points at which the fixing position temperatures (i.e., surface temperatures of the heat roller  51 ) are acquired during the sheet passing direction fixing temperature distribution sampling process of  FIG. 11 , and the acquired surface temperatures of the heat roller  51  may be each considered to be the surface temperature of the heat roller  51  (i.e., the fixing position temperature) at a position where the recording sheet touches the outer circumferential surface of the heat roller  51  at the corresponding sampling sheet position when each sampling sheet position passes the fixing position  53 . 
     Also, the present modification may also be applied to modifications (2) and (3) That is, during the one-side printing curl control process of  FIGS. 16 and 21 , the timing at which the sheet passing direction fixing temperature change detection process begins in step S 1605  may be earlier than the arrival of the leading edge of the recording sheet at the fixing position  53  by Δt. 
     (5) In the Embodiment, the image forming device is an image forming device that performs a secondary transfer of the unfixed image from the intermediate transfer belt to the recording sheet after performing the primary transfer of the unfixed image onto the intermediate transfer belt. However, image forming devices to which the Embodiment is applicable are, of course, not limited to image forming devices performing the secondary transfer. For example, the Embodiment may also be applied to an image forming device performing a direct transfer of the unfixed image from the photosensitive drum to the recording sheet.
 
(6) The change in fixing position temperature along the sheet passing direction during thermal fixing is not limited to occurring when temperature increase (step S 703 ) and temperature decrease (step S 707 ) are performed in the inter-page fixing temperature adjustment process of  FIG. 7 . For example, when performing thermal fixing of an initial page in an image formation process commenced after power ON, after a stand-by state, or the like, the thermal fixing of the initial page begins immediately after the surface temperature of the heat roller  51  is increased to the target temperature. As such, in such cases, the surface temperature of the heat roller  51  after the increase is not stable, and fluctuation in the surface temperature of the heat roller  51  during the thermal fixing of the initial page is greater than that during the thermal fixing of the second and subsequent pages. Thus, the fixing position temperature is prone to fluctuations.
 
     Accordingly, the both-side printing transfer voltage control process and the one-side printing curl control process of the Embodiment are also applicable to changes in fixing position temperature occurring in cases such as those described above. 
     (7) In the Embodiment, the applied transfer voltages at the sampling sheet positions are calculated using the transfer voltage formula. However, rather than using the transfer voltage formula, a table indicating a relationship between fixing position temperatures and applied transfer voltages may be created in advance (e.g., indicating the relationship between fixing position temperatures and applied transfer voltages at increments of 0.1° C. from the economy temperature to the upper limit temperature) and stored in the parameter storage unit  607 . The table may then be used to determine the applied transfer voltage at each sampling sheet position. 
     CONCLUSION 
     The image forming device pertaining to the aspect of the present disclosure described above is an image forming device capable of performing both-side printing with respect to a recording sheet, the image forming device statically transferring, by application of a transfer voltage, an unfixed image formed on an image carrier to the recording sheet when passing through a transfer position, and then thermally fixing the unfixed image onto the recording sheet when the recording sheet passes through a fixing position where a heating rotating body is disposed, the image forming device including: a water content index acquisition unit configured to acquire an index value of a water content at each of a plurality of sheet-passing-direction positions of the recording sheet having undergone thermal fixing of a first unfixed image statically transferred onto a first side thereof; and a transfer control unit configured to control, for each of the positions of the recording sheet, a transfer voltage applied for statically transferring a second unfixed image onto a second side of the recording sheet, so that the lower the water content indexed by the index value of the position, the greater an absolute value of the transfer voltage. 
     In the image forming device, the water content index acquisition unit may acquire the index value at each of the positions of the recording sheet by acquiring a temperature of the heating rotating body when the position of the recording sheet passes through the fixing position. 
     In the image forming device, a temperature applied while thermally fixing an unfixed image having been statically transferred onto the recording sheet may be controlled to change from a first temperature to a second temperature that differs from the first temperature. 
     In the image forming device, for each page to be printed, a target temperature at which a temperature of the heating rotating body is to be maintained while performing thermal fixing for the page may be determined according to image information for the page. 
     According to the above-described configuration, after the recording sheet has undergone thermal fixing of a first unfixed image statically transferred onto a first side thereof during both-side printing, the transfer voltage applied at each of the position of the recording sheet for statically transferring the second unfixed image onto the second side of the recording sheet is controlled so that the lower the water content indexed by the index value of the position, the greater the absolute value of the transfer voltage. As such, despite variations in water content within the recording sheet during both-side printing, the effect of fluctuations in electrical resistance caused by these variations is canceled out, thus enabling the static transfer of the second unfixed image onto the second side to be performed without distortion. As a result, degradation in image quality on the second side is prevented. 
     The image forming device may further include: a calculation unit configured to calculate, in one-side printing, an amount indicating a change in the water content in the recording sheet in the sheet passing direction based on the index value acquired at each of the positions by the water content index acquisition unit; a de-curling unit correcting a curl of the recording sheet; and a curl control unit causing the de-curling unit to correct the curl of the recording sheet when the amount exceeds a threshold. 
     Further, the image forming device pertaining to the aspect of the present disclosure described above may be an image forming device statically transferring, by application of a transfer voltage, an unfixed image formed on an image carrier to a recording sheet passing through a transfer position, and then thermally fixing the unfixed image onto the recording sheet when the recording sheet passes through a fixing position where a heating rotating body is disposed, the image forming device including: a water content index acquisition unit configured to acquire an index value of a water content at each of a plurality of sheet-passing-direction positions of the recording sheet having undergone thermal fixing of the unfixed image statically transferred onto the recording sheet; a calculation unit configured to calculate an amount indicating a change in the water content in the recording sheet in the sheet passing direction based on the index value acquired at each of the positions by the water content index acquisition unit; a de-curling unit correcting a curl of the recording sheet; and a curl control unit causing the de-curling unit to correct the curl of the recording sheet when the amount exceeds a threshold. 
     Accordingly, when the variation in water content in the sheet passing direction of the recording sheet after thermal fixing exceeds a threshold, and there is a risk that the recording sheet may experience curling, the curling caused by the variation in water content during thermal fixing is prevented. 
     In the image forming device, the curl control unit may control, in accordance with the amount, a degree to which the de-curling unit corrects the curl. 
     As a result, the degree to which the de-curling unit corrects the curl is adjusted in accordance with the change in water content in the recording sheet in the sheet passing direction, enabling optimization of the curl correction as neither too weak nor too strong. 
     The image forming device may further include a humidifier unit configured to humidify the recording sheet, and in the image forming device, when the amount exceeds the threshold and a grammage of the recording sheet is no greater than a predetermined lower limit of the grammage, the curl control unit may cause the humidifier unit to humidify the recording sheet and then causes the de-curling unit to correct the curl of the recording sheet. 
     As such, when the grammage of the recording sheet is equal to or less than a lower threshold and mechanical correction is insufficient to correct the curling, then the recording sheet is humidified after fiber resilience has been lowered in the recording sheet. Thus, insufficient curl correction is prevented from occurring, even in a thin recording sheet with low grammage. 
     Although the present disclosure has been fully described by way of examples with reference to the accompanying drawings, it is to be noted that various changes and modifications will be apparent to those skilled in the art. Therefore, unless such changes and modifications depart from the scope of the present disclosure, they should be construed as being included therein.