Patent Publication Number: US-8971742-B2

Title: Image forming apparatus to change image magnification by controlling sheet conveyance speed

Description:
This application is based on Japanese Patent Application No. 2011-053931 filed on Mar. 11, 2011, in Japanese Patent Office, the entire content of which is hereby incorporated by reference. 
     TECHNICAL FIELD 
     The present invention related to an image forming apparatus to fowl an electro latent image by exposing an image carrier, and to from an image onto a recording sheet by transferring a toner image obtained by developing the electrostatic latent image, and in particular, to control a conveyance speed of the sheet so as to change a magnification of the image. 
     BACKGROUND OF THE INVENTION 
     In the image forming apparatus to form an image by transforming the toner image onto the sheet, it is known that the sheet extends and contracts by effects of heat and pressure, and the size of the image formed on the sheet changes due to fluctuation of amounts of extension and contraction of the sheet in accordance with types of the sheets used. 
     There is widely used a method to change a moving speed of the image carrier as a method to adjust the magnification (hereinafter called a longitudinal magnification) of the image in a sheet conveyance direction. Namely, the longitudinal magnification is changed by changing the moving speed of the image carrier with respect to scan exposing carried out by a laser diode array or a light emitting diode array in a constant speed in a main scanning direction (a direction perpendicular to the moving direction of the image carrier) and in a sub-scanning direction (the moving direction of the image carrier). 
     In order to transfer the image formed on the image carrier onto the recording sheet, at a transfer position the recording sheet has to be conveyed in the same speed as that of the image carrier thus in order to change the lateral magnification the recording sheet conveyance speed is changed beside the image carrier moving speed. 
     Also, in order to match the sheet conveyance speed at a transfer position coincides and the image carrier moving speed, the conveyance speed of the sheet feeding section to feed the sheet to the transfer position is adjusted finely. As a driving section to drive the sheet feeding section, a stepping motor is usually used. By adjusting a frequency of a drive clock pulse to drive the stepping motor, fine adjustment of the conveyance speed of the sheet feeding section is conducted. 
     In Patent Document 1: Unexamined Japanese Patent Application Publication No. 2009-29003, there is a contraption that a condition not to lose steps of PLL is stored and read so that a rotation speed of a polygon mirror to conduct exposure in the main scanning direction is changed in a minimum amount of time required. 
     In Patent Document 3: Unexamined Japanese Patent Application Publication No. 2002-2039, besides changing a rotation speed of a photoconductive member, by changing a rotation speed of a register roller in accordance with the change of the rotation speed of the photoconductive member, the longitudinal magnification is changed. 
     Patent Document 1: Unexamined Japanese Patent Application Publication No. 2009-29003 
     Patent Document 3: Unexamined Japanese Patent Application Publication No. 2002-2039 
     As described above, to change the longitudinal magnification, the rotation speed of the polygon mirror and the photoconductive drum is changed and the moving speed of the intermediate transfer belt is changed. However it is known that the above changes are time consuming and decrease productivity. In particular, in a color image forming apparatus utilizing an intermediate transfer method, in order to match the obverse and reverse magnifications, when adjusting the obverse and reverse magnifications, adjusting periods are inserted between every obverse side and reverse side printing which drastically decrease the productivity. 
     Also, in case the intermediate transfer member and the secondary transfer member are in direct contact or pressed each other having the sheet in between without slippage, if the drive speed of the secondary transfer member is changed, there is a problem that the drive of the intermediate transfer member is interfered and misalignment of transfer and color shift occur. 
     SUMMARY 
     The present invention has one aspect to solve the above problems and an object of the present invention is to provide an image forming apparatus which controls longitudinal magnification while inhibiting misalignment of transfer due to a failure caused by a speed difference between the transfer member and the recording sheet, and speed differences among individual transfer members without requiring changing time to change the longitudinal magnification. 
     To achieve the above object, the image forming apparatus reflecting one aspect of the present invention are as follow.
     Item 1. An image forming apparatus, having: an image carrier to carry a toner image; a toner image forming section to form the toner image on the image carrier; an intermediate transfer member to transfer the toner image; a first transfer section to transfer the toner image on the image carrier onto the intermediate transfer member, and a second transfer member to conduct secondary transfer in which the toner image transferred by the first transfer section is transferred onto a sheet and convey the sheet, wherein the control section controls a magnification of the toner image transferred onto the sheet at a secondary transfer position in a sheet conveyance direction to be a target value by changing a rotation speed of the second transfer section while a rotation speed of the intermediate transfer member remains unchanged so as to change a conveyance speed of the sheet.   Item 2. The image forming apparatus of item 1, further having a memory section to store a conversion table in which the rotation speed of the second transfer section and the target value of the magnification of the toner image in the sheet conveyance direction correspond with each other, the control section controls the rotation speed of the second transfer section based on the conversion table.   Item 3. The image forming apparatus of item 1, further having a lubricant applying section to apply a lubricant, wherein the lubricant applying section apply the lubricant onto at least one surface of the intermediate transfer member or the second transfer section.   Item 4. The image forming apparatus of item 1, wherein a plurality of types of the sheet can be conveyed and the control section controls the rotation speed of the second transfer section based on the conversion table corresponding to the type of the sheet to be conveyed.   Item 5. The image forming apparatus of item 1, further comprising:   

     a registering section located at an upstream side of the second transfer section to adjust sheet conveyance timing when transferring onto the sheet and to transfer the sheet, and a fixing section located at a downstream side to fix the toner image on the sheet on which the toner image is transferred, wherein the control section controls at least a sheet conveyance speed of the register section or a sheet conveyance speed of the fixing section in accordance with the target value of the magnification of the toner image in the sheet conveyance direction.
     Item 6. The image forming apparatus of item 1 capable of forming both surfaces of the sheet, wherein the control section controls the rotation speed of the second transfer section when transferring the toner image on the obverse surface of the sheet and the rotation speed of the second transfer section when transferring the toner image on the reverse surface of the sheet respectively.   Item 7. The image forming apparatus of item 1, further comprising an input section to input the target value of the magnification of the toner image transferred onto the sheet in a sheet conveyance direction, wherein the control section acquires a value of the rotation speed of the second transfer section from the conversion table store in the memory section based on the magnification to be inputted to the input section.   

    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is an entire configuration of an image forming apparatus. 
         FIG. 2  is a cross-sectional view of an intermediate transfer member cleaning section  8 . 
         FIG. 3  is a cross-sectional view of a periphery of second transfer section. 
         FIG. 4  is a block diagram of a circuitry showing electrical configuration of the image forming apparatus. 
         FIGS. 5   a  and  5   b  are conversion tables of longitudinal image magnifications and secondary transfer speeds. 
         FIG. 6  is a flow chart to control a magnification of an image in a conveyance direction. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     While the present invention will be described based on the embodiments shown by the figures, the present invention is not limited to the embodiments thereof. Incidentally, the present invention can be applied to the image forming apparatus such as a printer not having an image reading section such as a scanner or an image forming apparatus such as a copying machine having the image reading section such as the scanner. 
     (Mechanical Configuration of Image Forming Apparatus) 
       FIG. 1  is an entire configuration of an image forming apparatus related to the present invention. The image forming apparatus is configured with an image forming apparatus GH and an image reading apparatus YS. 
     The image forming apparatus GH is so-called a tandem type color image forming apparatus configured with an image forming section  10  having a plurality of sets of toner image fanning sections  10 Y,  10 M,  10 C and  10 K as well as an inter mediate transfer belt  6  representing an intermediate transfer member in a shape of a belt, sheet conveyance device  20  and a fixing device  30 . 
     The image forming section  10  forms a toner image on a sheet P conveyed from the sheet feeding section  20  and convey the sheet P on which the toner image is formed to the fixing device  30 . 
     On an upper section of the image forming apparatus GH, the imge reading apparatus YS configured with an automatic document feeding device  60  and a document reading section  70  is installed. 
     A document d placed on a document table of the automatic document feeding device  60  is conveyed by the conveyance section and the image on one side or images on both sides of the sheet P are subject to scan exposure via an optical system of the document reading section  70  representing a document scan exposure section and read by a line image sensor CCD. 
     An analogue signal having been subject to photoelectric conversion by the line image sensor CCD is subject to analogue processing, A/D conversion, shading correction and image compression processing in an image processing section  101 , thereafter inputted to exposing sections (image writing section)  3 Y,  3 M,  3 C and  3 K 
     The toner image forming section  10 Y to form a yellow (Y) image is provided with a charging section  2 Y, an exposing section  3 Y, a developing section  4 Y and an image carrier cleaning section  5 Y which are disposed at a circumference of the photoconductive drum  1 Y representing an image carrier. The toner image forming section  10 M to form a magenta (M) image is provided with a charging section  2 M, an exposing section  3 M, a developing section  4 M and an image carrier cleaning section  5 M which are disposed at a circumference of the photoconductive drum  1 M representing an image carrier. The toner image forming section  10 C to form a cyan (C) image is provided with a charging section  2 C, an exposing section  3 C, a developing section  4 C and an image carrier cleaning section  5 C which are disposed at a circumference of the photoconductive drum  1 C representing an image carrier. The toner image forming section  10 K to form a black (K) image is provided with a charging section  2 K, an exposing section  3 K, a developing section  4 K and an image carrier cleaning section  5 K which are disposed at a circumference of the photoconductive drum  1 K representing an image carrier. The charging section  2 Y and the exposing section  3 Y, the charging section  2 M and the exposing section  3 M, the charging section  2 C and the exposing section  3 C, and the charging section  2 K and the exposing section  3 K configure latent image forming sections to form electrostatic latent images on the image carrier. 
     The developing sections  4 Y,  4 M,  4 C and  4 K store binary developers which are composed of small particle toners of yellow (Y), magenta (M), cyan (C) and black (K), and a carrier. 
     The images of respective colors formed by the toner image forming sections  10 Y,  4 M,  4 C and  4 K are sequentially transferred (primary transfer) onto the intermediate transfer belt  6  in rotation by the primary transfer sections  7 Y,  7 M,  7 C and  7 K so that a combined color image is formed. 
     While the toner image forming sections for yellow, magenta, cyan and black colors and the components thereof are distinguished by suffixes Y, M, C, and K, the suffixes are omitted except that distinguishing is necessary. 
     The primary transfer section  7  is configured with an unillustrated primary transfer roller and a spring to bias the primary transfer roller onto the intermediate transfer belt  6 . 
     The sheet P stored in a sheet feeding cassette  21  of the sheet feeding apparatus  20  is fed by the sheet feeding section  22  and conveyed by the sheet feeding rollers  23 ,  24 ,  25  and  26 . Then the sheet P stops once so as to register a frond end of the sheet P and the front end of the image, thereafter passes through the register roller  27  which adjusts timing and the sheet P is conveyed to the secondary transfer roller  9   a  where the color image is secondarily transferred onto the sheet P. The register roller  27  serves a function of a register section. 
     The secondary transfer roller  9   a  is a conductive transfer roller to which a transfer voltage is applied by an unillustrated power source. 
     The secondary transfer roller  9   a  configures the secondary transfer section  9  to transfer the toner image from the intermediate transfer belt  6  onto the sheet P. 
     The sheet P on which the color image is transferred is nipped by the fixing device  30  and by applying heat and pressure the color toner image on the sheet P is fixed onto the sheet P. Then nipped by the sheet ejection rollers  28 , the sheet P is placed on a sheet ejection tray  29  outside the apparatus. 
     On the other hand, after transferring the color image onto the sheet P via the secondary transfer section  9 , the intermediate transfer belt  6  from which the sheet P is separated by self stripping is clean by the intermediate transfer member cleaning device  8  to remove residual toner. 
     In case the sheet P having been subject to fixing processing is reversely ejected, the sheet P passes through a conveyance path located on the right side of a bifurcation plate  28 A in the figure disposed between the fixing device  30  and the sheet ejection roller  28 , then after being conveyed to a first conveyance path Pa at a lower portion, the sheet P is reversely conveyed and passes through a second conveyance path Pb on the left side of the bifurcation plate  28 A in the figure so as to be ejected outside the apparatus via the ejection roller  28 . 
     In case of copying on both sides of the sheet P, after fixing the image formed on the first surface (obverse surface) of the sheet P, the sheet P enters into the first conveyance path Pa then further enters into a fourth conveyance path Pd under the bifurcation plate  28 B, thereafter the sheet P is reversely conveyed so as to pass through the conveyance path on the right side of the bifurcation plate  28 B and conveyed to a third conveyance path Pc. Then the sheet P is diverted upward to be conveyed by the sheet feeding roller  26 . The toner image forming sections  10 Y,  10 M,  10 C and  10 K form respective colors of the images on the second surface (reverse surface) of the sheet P and the sheet P is subject to heat fixing processing via the fixing device  30  then ejected outside the apparatus via sheet ejection roller  28 . 
       FIG. 2  is a cross-sectional view of the intermediate transfer member cleaning device  8 . 
     A cleaning blade  8   a  configured with an elastic member in a plate shape is in contact with the intermediate transfer belt  6  at an edge towards an opposite direction (counter method) to a rotation direction of the intermediate transfer belt  6 . As the cleaning blade  8   a , for example, a urethane rubber attached to a support metal plate  81   a  is used. 
     At a downstream side of the cleaning blade  8   a  in the rotation direction of the intermediate transfer belt  6  and under the intermediate transfer belt  6 , there is disposed a lubricant applying device  8   b  to apply a lubricant to the intermediate transfer member. The lubricant application device  8   b  is configured with a brush roller  82   b , a solid form lubricant  83   b , a support guide  86   b  to retain the lubricant  83   b , a push up table  84   b  which slides in the support guide  86   b  and a pressing spring  85   b  to make the lubricant  83   b  in contact with the brush roller  82   b  via the push up table  84   b  under a predetermined pressure. As materials of the lubricant  83   b , for example, a zinc stearate (ZnSt) having a hardness equivalent to pencil hardness of HB is used. By rotating the brush roller  82   b  the lubricant  83   b  is scraped off and adheres on ends of bristles of the brush and by contacting the brush with the surface of the intermediate transfer belt  6 , fine powder of the lubricant  83   b  scraped off is applied to the intermediate transfer belt  6 . 
     The brash roller  82   b  is rotated and driven by an unillustrated motor whose rotation speed is variable. By increasing the rotation speed, an amount of the fine power of the lubricant increases and by decreasing the rotation speed, the amount of the fine power of the lubricant decreases. 
     A leveling section  8   d  in contact with the surface of the intermediate transfer belt  6  is disposed at a further downstream side of the lubricant application device  8   b . The leveling section  8   d  configured with a blade formed by an elastic member in a plate shape is in contact with the intermediate transfer belt  6  at an edge towards the same direction as the rotation direction of the intermediate transfer belt  6 . By disposing the leveling section  8   d  there is an effect that the lubricant applied by the lubricant application device  8   b  is pressed onto the intermediate transfer belt  6  and it is possible that the thickness of the lubricant is controlled and adhesion with respect to the surface thereof is enhanced. 
       FIG. 3  is a cross-sectional view of a periphery of second transfer section. 
     The lubricant application device  9   b  to apply the lubricant to the second transfer member is configured with a brush roller  92   b , a solid form lubricant  93   b , a support guide  96   b  to retain the lubricant  93   b , a push up table  94   b  which slides in the support guide  96   b  and a pressing spring  95   b  to make the lubricant  93   b  in contact with the brush roller  92   b  via the push up table  94   b  under a predetermined pressure. As materials of the lubricant  93   b , for example, a zinc stearate (ZnSt) having a hardness equivalent to pencil hardness of HB is used. By rotating the brush roller  92   b  the lubricant  93   b  is scraped off and adheres on ends of bristles and by contacting the brush with the surface of the second transfer roller  9   a , the fine powder of the lubricant  93   b  scraped off is applied to the second transfer roller  9   a.    
     The brash roller  92   b  is rotated and driven by an unillustrated motor where rotation speed is variable. By increasing the rotation speed, an amount of the fine power of the lubricant to be applied to the second transfer roller  9   a  increases and by decreasing the rotation speed, the amount thereof decreases. 
     A leveling section  9   d  in contact with the surface of the second transfer roller  9   a  is disposed at a further downstream side of the lubricant application device  9   b . The leveling section  9   d  configured with a blade formed by an elastic member in a plate shape is in contact with the second transfer roller  9   a  at an edge towards the same direction as the rotation direction of the second transfer roller  9   a . By disposing the leveling section  9   d  there is an effect that the lubricant applied by the lubricant application device  9   b  is pressed onto the second transfer roller  9   a  and it is possible that the thickness of the lubricant is controlled and adhesion with respect to the surface thereof is enhanced. 
     The intermediate transfer belt  6  supported by the intermediate transfer roller  6   a  and the second transfer roller  9   a  are in contact with each other. When the sheet P is nipped and conveyed to the fixing device  30 , by the effect of the lubricant from the lubricant application device  9   b  the sheet P is conveyed while the sheet P is slipping with respect to the second transfer roller  9   a . In the same manner, by the effect of the lubricant from the aforesaid lubricant application device  8   b , the sheet P I conveyed while the sheet P is slipping with respect to the intermediate transfer belt  6 . 
     The lubricant application device  8   b  and the lubricant application device  9   b  serve a function of the lubricant application section of the present invention. 
     Incidentally, in the description of the image forming apparatus GH, while color image forming has been described, the monochrome image forming is included in the present invention. 
     (Electrical Configuration of Image Forming Apparatus) 
       FIG. 4  is a block diagram showing an electrical configuration of an image forming apparatus. The automatic document feeding device  60  is provided with an ADF control section  68  to control the unillustrated drive section. The document reading section (scanner section)  70  is provided with a line image sensor  76  and a scanner control section  79 . The scanner control section  79  conducts control of on and off of a light source and control of moving a scan exposing section 
     An operation control section  50  is provided with a display section  51  configured with a liquid crystal display (LCD), an operation section  52  configured with touch switches located on a screen thereof and switches and an operation control section  53  to control operation of the above sections. 
     The printer section  40  is provided with a laser unit  42  and a printer control section  41 . Also, the printer control section  41  is provided with functions to control operations of a charging device  2 , a primary transfer section  7 , voltage application to a separation device, a developing device  4 , a second transfer section  9 , an intermediate transfer member cleaning device  8 , a fixing device  30  and a sheet feeding conveying device  20 . The printer control section  41  serves a function of control section of the present invention. 
     Also, the printer control section  41  is provided with a second transfer motor  43 M to rotate the second transfer roller  9   a , a second transfer roller drive section  43  which controls the second transfer motor  43  M to rotate at a predetermined rotation speed, a register roller motor  44 M to rotate a register roller  27 , a register roller drive section  44  to start rotation of the register roller motor  44 M in a way that timing of front end of the sheet coincides with timing of the front end of the image coincide, and perform conveyance control of the sheet P at a predetermined speed a fixing roller motor  45  M disposed in the fixing device  30  to rotate the fixing roller  31 , and a fixing roller drive section  45  which controls the fixing roller motor  45  M so as to rotate at a predetermined speed. 
     The ADF control section  68 , the scanner control section  79 , the operation control section  53  and the printer control section  41  are configured with respective circuitries having CPUs, ROMs and RAMs as main components. Various kinds of controls are executed in accordance with programs stored in the ROMs. 
     The total control section  100  serves a function to conduct overall control of operation of the image forming apparatus GH. The total control section  100  is provided with a reading process section  101 , a DRAM control IC  102 , a compression/extension IC  103 , an image memory  104 , a writing process section  105 , an image control CPU  110 , a program memory  106 , a system memory  107 , a nonvolatile memory  108  representing a memory section of the present embodiment and an I/O port  109 . 
     The reading process section  101  servers functions of enlargement processing, mirror image processing binarization process by error diffusion with respect to the image data outputted from the document reading section (scanner section)  70 . The compression/extension IC  103  serves a function in compressing a binarized image data or extending a compressed image data. The image memory  104  serves a function as a page memory capable of storing non-compressed image data by pages and a function as a compression memory  104   a  to accumulate compressed image data. 
     The writing process section  105  serves a function to transmit extended image data read out from the image memory  104  to a laser unit  42  configured with exposing sections  3 Y,  3 M,  3 C and  3 K with timing corresponding to operation of the printer section  40 . DRAM control IC  102  conducts timing control of reading/writing and refreshing of the image memory  104  configured with a dynamic RAM, compressing and storing the image data in the image memory  104 , and reading the compressed data from the image memory  104  and extending it. 
     The image control CPU  110  is a CPU to control entire operation of the image forming apparatus GH and serves functions to control flow of the image data and to control entry of a programmed job and execution of the job. The program memory  106  is a memory in which a program to be executed by the image control CPU  110  is stored and a system memory  107  is a work memory to temporally store various kinds of data while the program is being executed. The image control CPU  110  controls entire image forming apparatus GH while conducting serial communication with the printer control section  41  in accordance with the program stored in the program memory  106  with reference to data in the system memory  107 . 
     Nonvolatile memory  108  is a memory to store user data and system data to be store even after turning off the power. To the I/O port  109  various kinds of sensors such as sheet size detection sections to detect sheet sizes set in respective sheet feeding trays and LED elements are connected. 
     Before starting image forming, in a setting predetermined mode, a key operator or a user sets sheet size and sheet type of the sheets to be stacked in respective sheet feeding trays via the operation display section  50  in accordance with a sheet setting screen of the sheet tray displayed on the display section  51  by the image control CPU  110  the above conditions are stored in the nonvolatile memory  108  by the image control CPU  110 . 
     Incidentally, while the control of the control section of the present embodiment will be described as a function of the printer control section  41 , it can be functions of the total control section  100  and the printer control section  41  or a function of the total control section  100 . Also, in the same manner as the total control section  100 , the printer control section  41  is provided with an unillustrated control CPU, a program memory, a system memory and a nonvolatile memory. 
     From here, there will be specifically described control of magnification of the image to be transfer at the secondary transfer position in the conveyance direction by changing the rotation speed of the second transfer roller  9   a  representing the second transfer section of the present embodiment. 
     The register roller  27  is to match a front end of the sheet with a front end of the image by synchronizing timing by stopping the sheet once. The sheet P conveyed from the register roller  27  is nipped by an intermediate transfer belt  6 , which is supported rotatably by intermediate transfer rollers  6   a , and the secondary transfer roller  9   a  to be conveyed simultaneously when a toner image carried by the intermediate transfer belt  6  is secondarily transferred onto the sheet P. 
     When the sheet P is conveyed in the secondary transfer section  9 , the sheet P slips with respect to the intermediate transfer belt  6  since the lubricant is applied on the intermediate transfer belt  6  by the lubricant application device  8   b  and the sheet P slips with respect to the secondary transfer roller  9   a  since the lubricant is applied on the surface of the secondary transfer roller  9   a  by the lubricant application device  9   b.    
     By increasing the rotation speed of the secondary roller  9   a  while keeping the speed of the intermediate transfer belt  6  constant, the sheet P is conveyed faster than the rotation speed of the intermediate transfer belt  6 , as a result the toner image transferred onto the sheet P is enlarged in the conveyance direction. By decreasing the rotation speed of the secondary transfer roller  9   a  compared to the speed of the intermediate transfer belt  6 , the sheet P is conveyed slower than the rotation speed of the intermediate transfer belt  6 , as a result the toner image transferred onto the sheet P is contracted in the conveyance direction. 
     An amount of enlarging and contracting of the image to be transferred onto the sheet P created by the speed difference between the intermediate transfer belt  6  and the secondary transfer roller  9   a  varies with a plurality of types of the sheet P capable of being conveyed, for example, sheet thickness.  FIGS. 5   a  and  5   b  show examples of a conversion tables in which the target values of longitudinal magnification of the image correspond to the secondary transfer speeds. 
       FIG. 5   a  is a conversion table in case of a thick sheet having the basis weight of 300 g/m 2  and the  FIG. 5   b  is a conversion table in case of a thin sheet having the basis weight of 64 g/cm 2 . 
     Also, in order to match the image sizes of the image on the obverse side which has been passed through the fixing device and the size of the image of the reverse side, the rotation speed of the secondary transfer roller  9   a  when transferring on the obverse side and the rotation speed of the secondary transfer roller  9   a  when transferring on the reverse side are controlled respectively. 
     By according the rotation speed of the register roller  27  and the fixing roller  31  with the sheet conveyance speed, the conveyance of the sheet P can be stabilized. 
       FIG. 6  shows a flow chart to control the magnification of the image in the conveyance direction. 
     Step  200  judges whether or not the target value of the magnification of the image in the conveyance direction exists. In case the judgment result is “No”, the operation procedure proceeds to Step  230  and in case it is “Yes” the operation procedure proceeds to Step  210 . 
     In Step  210 , the target value of the magnification image is inputted by inputting values via the operation section  52  representing an input section of the present embodiment by an operator or by a maintenance and service staff, or by transmitting values from external devices such as a PC and a network via I/O port  109 . Here, a magnification M 1  of the obverse side and a magnification M 2  of the reverse side can be set. 
     The conversion table shown by  FIGS. 5   a  and  5   b  stored in the nonvolatile memory  108 . In accordance with the inputted magnification M 1  of the obverse side and the magnification M 2  of the reverse side, a rotation speed coefficient R 1  to determine the secondary transfer roller at conveyance of the obverse side and a rotation speed coefficient R 2  of the secondary transfer roller at conveyance of the reverse side are set from the conversion table in the nonvolatile memory  108  (Step  220 ). 
     For example, in case the sheet subject to secondary transfer is the thick sheet having the basis weight of 300 g/m 2 , the magnification of the obverse side is increased by 0.2%, thus +5% of the rotation speed coefficient R 1  of the secondary transfer roller  9   a  obtained from the conversion table in  FIG. 5   a  is set, and in case the magnification of the reverse side is not changed, ±0% of the rotation speed coefficient R 2  of the secondary transfer roller  9   a  obtained from the conversion table in  FIG. 5   a  is set. 
     R 1  and R 2  having been set are stored in the nonvolatile memory  108  so that the same rotation speed coefficient of the secondary transfer roller is applied whenever the thick sheet of 300 g/m 2  is selected. 
     In Step  230 , the operation procedure waits until a printing command is received. In case printing command is “Yes” the operation procedure proceeds to a next step. 
     In Step  240 , printing on the obverse side starts. The printer control section  41  drives the secondary transfer motor  43 M by controlling the secondary transfer roller drive section  43  in accordance with the rotation speed coefficient R 1  of the secondary transfer roller. At the same time, the printer control section  41  drives a register roller motor  44 M and a fixing roller motor  45 M by setting a drive speed coefficient R 1   a  of the register roller and a drive speed coefficient R 2   b  of the fixing roller, wherein the longitudinal image magnification of the sheet corresponding to the rotation speed coefficient R 1  of the secondary roller, namely the target value of the magnification of the image in the conveyance direction of the secondary transfer is the speed coefficient. 
     As the aforesaid example, in case of the thick sheet having the basis weight of 300 g/m 2 , the rotation speed coefficient R 1  of the secondary transfer roller  9   a  is +5% and a target value of the longitudinal image magnification from the conversion table in  FIG. 5   a  is applied, and the drive speed coefficient Rla of the register roller  27  is set to be +0.2% and the drive speed coefficient Rib of the fixing roller  31  is set to be +0.2%. Whereby, in case the secondary transfer is carried out while the sheet is being pressed and conveyed by the register roller and while the sheet is being pressed and conveyed by the fixing roller, the conveyance speed of the sheet can be stabilized. 
     In case the secondary transfer onto the obverse side is completed in Step  250  (the result of judgment is “Yes”), the operation procedure proceeds to a next step. 
     In Step  260 , printing of reverse side starts, the secondary transfer roller  9   a  is driven in accordance with the rotation speed coefficient R 2  of the secondary transfer roller  9   a , and the drive speed coefficient R 2   a  of the register roller  27  corresponding to the rotation speed coefficient R 2  of the secondary transfer roller  9   a  and the drive speed coefficient R 2   b  of the fixing roller  31  are set, then the register roller motor  44 M and the fixing roller motor  45 M are driven. 
     As the aforesaid example, in case the magnification of the reverse side is not changed, with reference to the conversion table in  FIG. 5   a , the rotation speed coefficient R 2  of the secondary transfer roller  9   a  is set to be ±0% so as to conduct secondary transfer. 
     In case the secondary transfer for the reverse side is completed in Step  280 , (judgment result is “Yes”), the processing procedure proceeds to the next step. 
     The Step  240  to the Step  280  are repeated until printing of a designated number of the sheets is completed, wherein the designated number is set by the operator via the operation section  52  or by the PC or the network via the I/O port  109 , and operation is terminated when the printing of the designated number of the sheets is completed. 
     As above, since the lubricant is applied by the lubricant application device Sb on the intermediate transfer belt  6 , the intermediate transfer belt  6  and the sheet P slips each other and a conveyance force is created. Also, since the lubricant is applied by the lubricant application device  9   b  on the surface of the secondary transfer roller  9   a , the secondary transfer roller  9   a  and the sheet P slip each other and a conveyance force is created. Whereby, in the secondary transfer section  9  the image can be enlarged or contracted when the image is transferred from the intermediate transfer belt. 
     Incidentally, the in case of printing only on the obverse surface the Steps  260  and  270  in the flow chart are omitted. 
     Also, in an apparatus where the image size differs in the conveyance direction on the sheet after transferring due to a difference of printing rate of the image transferred onto the sheet, the image size on the sheet can be adjusted by performing the same control as above. 
     As above the intermediate transfer belt  6  supported by the intermediate transfer roller  6   a  is in contact with the secondary transfer roller  9   a . When the sheet P is nipped and conveyed to the fixing device  30 , by the effect of the lubricant applied by the lubricant application device  9   b , the sheet P is conveyed while slipping with respect to the secondary transfer roller  9   a . Whereby, by changing the rotation speed of the secondary transfer roller  9   a , the magnification of the image to be transferred onto the sheet in the conveyance direction can be changed. 
     Also, the present invention includes a secondary transfer section using a transfer belt supported and suspended by a plurality of the supporting rollers which is substitution of the secondary transfer roller  9   a.    
     By changing the rotation speed of the secondary transfer section, the longitudinal magnification can be changed while prohibiting misalignment of transfer due to failure of drive caused by speed differences between the transfer member and sheet, and among each of transfer members without requiring changing time to change the longitudinal magnification which decreases productivity.