Patent Publication Number: US-7583282-B2

Title: Image printing apparatus and image printing method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application makes reference to, incorporates the same herein, and claims all benefits accruing under 35 U.S.C. §119 from applications for IMAGE PRINTING APPARATUS AND IMAGE PRINTING METHOD earlier filed respectively in the Japanese Patent Office on Jan. 5, 2006 and Jun. 28, 2006, and duly assigned with the applications Nos. 2006-000624 and 2006-177725. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an image printing apparatus such as a copying machine, a printer, a facsimile apparatus, and a multifunction peripheral of a copying machine, printer, and facsimile apparatus and, more particularly, to an image printing apparatus and image printing method which can register the positions of images to be printed on the front and back surfaces of a sheet highly accurately. 
   2. Description of Related Art 
   An electrophotographic image printing apparatus comprises a photosensitive member, an image write unit, developing portion, feeder, transfer portion, fixing unit, and the like, and can print images on the two surfaces of a sheet. 
   When printing images on the two surfaces of the sheet, the images on the two surfaces must be registered accurately. This is to prevent the following problems. For example, when a bundle of sheets P are cut or bound, if images printed on the front and back surfaces are misregistered, a blank may be left depending on the images, or the images may partly lack after cutting. 
   Conventionally, a mark is printed on the front surface of the sheet P, and the position of the mark is detected to correct the image-printing position on the back surface (for example, see Japanese Unexamined Patent Publication No. 10-319674 (patent reference 1)). 
   In an image printing apparatus which employs thermal fixing, when an image is printed and fixed on the front surface, the sheet P after fixing shrinks to shrink the image simultaneously. If an image is printed and fixed on the back surface in the same manner, the positions of the images printed on the front and back surfaces are misregistered. The method described in patent reference 1 is aimed at correcting the skew of the sheet P caused by a convey error or the like, and shrinkage of the image caused by the shrinkage of the sheet P due to the fixing process is not taken into account. Although the positions of the distal ends of the images on the front and back surfaces may be registered, it is impossible to set the positions and sizes of the images on the front and back surfaces to coincide with each other highly accurately. 
   Hence, in the image printing apparatus which employs thermal fixing, as the sheet P shrinks as described above, to obtain images on the front and back surfaces that coincide with each other, the position and size of the image to be printed on the back surface must be corrected. 
   Attempts have been made to print an image on the back surface considering the shrinkage of the sheet caused by the fixing process described above (for example, see Japanese Unexamined Patent Publication No. 2003-156974 (patent reference 2)). In the image printing apparatus described in patent reference 2, marks are printed at four corners on the front surface of a sheet P or at two portions in a direction (to be referred to as the “main scanning direction” hereinafter) perpendicular to the convey direction (to be referred to as the “sub-scanning direction” hereinafter) of the sheet P. The distances between the marks or the like before and after fixing the image on the front surface are obtained. The position and size of the image on the back surface are determined on the basis of the distances or the like. 
   With the image printing apparatus described in the above patent reference 2, however, the marks are printed at the four corners of the sheet P or at the two portions in the main scanning direction, and the marks serve as cutting marks used as marks in cutting the sheet P, or as color misregistration correction marks used in correction of color misregistration of the images. Thus, the following problems arise. 
   Since the marks are printed at the four corners of the sheet P or at the two portions in the main scanning direction, a large sensor detection range must be set, or a plurality of sensors must be provided. In order to arrange a one-dimensional line sensor in the main scanning direction to obtain the shrinkage factor in the convey direction of the sheet P, the entire sheet P must pass through the one-dimensional line sensors so the one-dimensional line sensor detects all the marks printed at the four corners or the like. Then, however, the shrinkage factor cannot be obtained until the sheet P has passed through the one-dimensional line sensor, and mark detection is delayed. To feedback the detection result of the one-dimensional line sensor to back surface image printing, the shrinkage factor must be obtained since the marks are detected until back surface image printing to correct the position and size of the image on the back surface. Because mark detection is delayed, the image printing section and the mark detection position, i.e., the position to set the one-dimensional line sensor, must be spaced apart from each other. In this manner, with the prior art, the position to set the sensor is limited. 
   To detect and recognize a cutting mark or color misregistration correction mark with the line sensor, the line sensor must read the mark quickly and frequently. A large-capacity memory is also necessary to store data read by the line sensor. 
   SUMMARY OF THE INVENTION 
   The present invention has been made to solve the problems described above, and can provide an image printing apparatus and image printing method which, when printing images on the two surfaces of a sheet, can register the positions and sizes of images to be printed on the front and back surfaces of the sheet highly accurately. 
   According to the present invention, there is provided an image printing apparatus for printing images on two, front and back surfaces of a sheet, comprising an image printing section which forms the image for the front surface of the sheet as well as an image of one reference mark on a photosensitive member in a region outside an image printing region and transfers the images formed on the photosensitive member onto the front surface of the sheet, a fixing unit which fixes the images on the sheet, a line sensor which detects the reference mark after the image on the front surface is fixed and before the image is printed on the back surface, an arithmetic operating section which obtains a shrinkage factor of the sheet on the basis of a size of the reference mark before fixing and a size of the detected reference mark after fixing and calculates a position and magnification of the image to be printed on the back surface on the basis of the shrinkage factor, and a control section which performs control operation to print the image on the back surface of the sheet on the basis of the calculated position and the calculated magnification. 
   According to the present invention, there is also provided an image printing method of printing images on two, front and back surfaces of a sheet, comprising the first image printing step of forming the image for the front surface as well as an image of a reference mark at one portion on a photosensitive member in a region outside an image printing region and transferring the images formed on the photosensitive member onto the front surface of the sheet, the fixing step of fixing the images onto the sheet, the detection step of detecting the reference mark after fixing which is printed on the front surface of the sheet with a line sensor, the arithmetic operation step of obtaining a shrinkage factor of the sheet on the basis of a size of the reference mark before fixing and a size of the detected reference mark after fixing and calculating a position and magnification of the image to be printed on the back surface on the basis of the shrinkage factor, and the second image printing step of printing the image on the back surface of the sheet on the basis of the calculated position and the calculated magnification. 
   According to the present invention, when printing the images on the two surfaces of the sheet, the position and size of the image on the front surface can be registered with those of the image on the back surface highly accurately. 
   According to the present invention, the shrinkage factor of the sheet can be obtained by only printing a reference mark at one position on the front surface to calculate the position and size of the image on the back surface. 
   Furthermore, according to the present invention, since the reference mark is printed at one portion on the front surface, it is possible to narrow the detection region of the line sensor more than in the prior art. Thus, the reference mark can be detected within a shorter period of time, and the amount of detected data can be small. As the mark can be detected within the short period of time, the mark can be detected immediately before printing the image on the back surface to calculate the position and size of the back surface image, so that the correction accuracy of the position and size can improve. 
   The present invention is more specifically described in the following paragraphs by reference to the drawings attached only by way of example. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A more complete appreciation of the present invention, and many other attendant features and advantages thereof, will become apparent as the same becomes better understood by reference to the following detailed description when considered in conjunction with the accompanying drawings in which like reference symbols and numerals indicate the same or similar components, wherein: 
       FIG. 1  is a sectional view showing the schematic entire arrangement of an image printing apparatus according to the present invention; 
       FIG. 2  is a view showing an example of a method of determining the position of an image to be printed on the front surface of a sheet; 
       FIG. 3  is a view showing an image and reference mark printed on the front surface of the sheet; 
       FIG. 4  is a view showing a reference mark printed on the front surface of the sheet; 
       FIG. 5  is a view showing an example of a method of detecting a mark when printing an image on the back surface of the sheet; 
       FIG. 6  is a control block diagram of an image printing apparatus according to the first embodiment; 
       FIG. 7  is a view for explaining a process of obtaining the position and magnification of an image to be printed on the back surface; 
       FIG. 8  is a flowchart showing the control operation of the image printing apparatus according to the first embodiment; 
       FIG. 9  is a control block diagram of an image printing apparatus according to the second embodiment; 
       FIGS. 10A to 10C  are views for explaining a process of obtaining the position and magnification of the image to be printed on the back surface; and 
       FIG. 11  is a flowchart showing the control operation of the image printing apparatus according to the second embodiment. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   Some preferred embodiments of the present invention will be described below in detail with reference to the accompanying drawings. 
   First Embodiment 
   The arrangement of an image printing apparatus according to the first embodiment of the present invention will be described with reference to  FIG. 1 .  FIG. 1  is a sectional view showing the arrangement of the image printing apparatus. 
   Reference numeral  10  denotes an automatic document feeder which conveys a document to read it. A plurality of documents d each with its first page facing up are placed on a document support  11  where the document is to be placed. The document d is fed out through rollers  12   a  and  12   b  and conveyed to an image reader  20  through a roller  13 . The document d, the image of which is read by the image reader  20 , is delivered to a delivery plate  16 . 
   The image reader  20  optically scans the document d to generate image data. A light source  23  irradiates the document surface of the document d, and reflected light from the document d forms an image on the light-receiving surface of a CCD  28  serving as a photoconverting means through mirrors  24 ,  25 , and  26  and a coupled optical system  27 . When the document d placed on a platen glass  21  with its read surface facing down is to be read, the optical system  27  reads it by scanning it along the platen glass  21 . When the document d is to be read while being conveyed, it is read with the light source  23  and mirror  24  being fixed under a second platen glass  22 . The readout image data of the document d is sent from the CCD  28  to an image processor (not shown). When the document d is to be duplex-conveyed by the automatic document feeder  10 , after its front surface is read, the document d is reversed and conveyed by reversing rollers  14  and conveyed to the roller  13  again. The back surface of the document d is read by the image reader  20 . The read image data is sent from the CCD  28  to the image processor. 
   Sheets P are stacked on a feed tray  30 . In  FIG. 1 , the feed tray  30  has only one stage. Alternatively, a plurality of stages of feed trays may be provided to stack sheets having different sizes. 
   A feeder  40  feeds the sheet P from the feed tray  30  to an image printing section  60 . The sheet P is fed out from the feed tray  30  by convey rollers  41  and abutted against the nip portion of registration rollers  43  through loop rollers  42  to stop temporarily, so the skew of the sheet P with respect to the convey direction is corrected. Then, the sheet P is conveyed to a transfer portion  63  at a predetermined timing. Alternatively, the sheet P is fed out from a manual feed tray  31  by convey rollers  44  and conveyed to the transfer portion  63  via the same process. 
   An image write unit  50  forms an electrostatic latent image on a photosensitive member  61  of the image printing section  60  on the basis of the image data of the document d which is read by the image reader  20 . A laser beam from a laser diode  51  corresponding to the image data irradiates the photosensitive member  61  of the image printing section  60  to form the electrostatic latent image. 
   The image printing section  60  prints an image on the sheet P in accordance with electrophotography. First, the laser beam from the laser diode  51  of the image write unit  50  irradiates the photosensitive member  61  which is uniformly charged by a charging portion  67 , to form an electrostatic latent image. The electrostatic latent image formed on the photosensitive member  61  is developed by a developing portion  62  to form a toner image on the photosensitive member  61 . The toner image is transferred onto the sheet P by the transfer portion  63  arranged under the photosensitive member  61 . The sheet P abutted against the photosensitive member  61  is separated by a separating portion  64 . The sheet P separated from the photosensitive member  61  is conveyed by a convey mechanism  65  to a fixing unit  70 . 
   The fixing unit  70  fixes the toner image transferred onto the sheet P with heat and pressure. 
   A delivery unit  80  delivers the sheet P on which the image is printed. The sheet P printed with the image is delivered by delivery rollers  81  onto a delivery tray  82 . When duplex image printing is to be performed, after an image is printed on the front surface, the sheet P is conveyed downward by a guide  83  and sent to a reversal path  84 . The sheet P entering the reversal path  84  is reversed by reversal convey rollers  85  and sent to a reversal convey path  86 . The sheet P entering the reversal convey path  86  is sent to the image printing section  60  again via the feeder  40 . 
   The sheet P is then abutted against the nip portion of the registration rollers  43  through the loop rollers  42  to stop temporarily, so the skew of the sheet P with respect to the convey direction is corrected. After that, the sheet P is conveyed to the transfer portion  63  at a predetermined timing. 
   In the image printing section  60 , a cleaning portion  66  removes the toner remaining on the photosensitive member  61  to prepare for the next image printing. In this state, the sheet P is loaded in the transfer portion  63  to print an image on its back surface. The sheet P separated from the photosensitive member  61  by the separating portion  64  is sent to the fixing unit  70  again through the convey mechanism  65  to fix the toner image on it. In this manner, the sheet P printed with the images on its front and back surfaces is delivered onto the delivery tray  82  by the delivery rollers  81 . 
   In the present invention, the convey direction of the sheet P may be referred to as a sub-scanning direction, and a direction perpendicular to the convey direction of the sheet P may be referred to as a main scanning direction. 
   In the image printing apparatus according to the first embodiment, a sheet detection sensor S 1  and line sensor S 2  are arranged between the registration rollers  43  and image printing section  60 . The sheet detection sensor S 1  detects the leading edge of the sheet P. The line sensor S 2  reads and detects a reference mark M (to be described later). 
   As shown in  FIG. 2 , the sheet P is sent by the registration rollers  43  in the convey direction at a predetermined timing to be conveyed to the image printing section  60 . In the image printing section  60 , in printing an image on the front surface of the sheet P, when the sheet detection sensor S 1  detects the leading edge of the sheet P, an image is printed on the sheet P at a position preset with reference to the leading edge of the sheet P. 
   For example, as shown in  FIG. 3 , the image is printed within an image region on the basis of image data with reference to the leading edge of the sheet P. Images of cutting marks K 1  to K 4  are printed at preset positions outside the image region. The cutting marks K 1  to K 4  serve as marks when cutting the sheet P. The sheet P is to be cut along the cutting marks K 1  to K 4 . The image of the reference mark M is printed at a preset position outside the image region which is outside the cutting marks K 1  to K 4 . According to the first embodiment, the reference mark M is printed at one portion outside the image region which is further outside the cutting marks in the main scanning direction and sub-scanning direction. The reference mark M serves as a reference in determining the position and size of an image to be printed on the back surface of the sheet P. 
   The reference mark M will be described with reference to  FIG. 4 .  FIG. 4  is a view showing the reference mark M printed on the front surface of the sheet P. The reference mark M consists of two, first and second straight lines L 1  and L 2  which are parallel to the sub-scanning direction, and a third straight line L 3  which is oblique to the sub-scanning direction. In printing the reference mark M, the lengths of L 1  and L 2  are equal. One end of L 3  is connected to the upstream end of L 1  in the convey direction, and the other end of L 3  is connected to the downward end of L 2  in the convey direction. In other words, the reference mark M has a Z shape. 
   Assume that each of L 1  and  2  has a length X 1  in the sub-scanning direction, and that the length between L 1  and L 2  is Y 1 . Assume that the intersection point of L 1  and L 2  is determined as a reference point PA, and the intersection point of L 2  and L 3  is determined as a reference point PB. The reference point PA serves as a reference in determining the image region of the back surface. The reference point PA in printing the mark M is printed at a position which is at a distance a from the trailing edge of the sheet P and at a distance b from the side surface of the sheet P. The size of the reference mark M in mark printing and the position to print the reference mark M are preset and stored in a storage section  3 . Regarding the size, the lengths X 1  and Y 1  are stored in advance. Regarding the position, the distances a and b which define the position of the reference point PA are stored in advance. The length X 1  corresponds to the “second length before fixing” of the present invention, and the length Y 1  corresponds to the “first length before fixing” of the present invention. The reference point PA corresponds to the “first reference point” of the present invention, and the reference point PB corresponds to the “second reference point” of the present invention. 
   In the first embodiment, one end of L 3  is connected to the upstream end of L 1  in the convey direction, and the other end of L 3  is connected to the downstream end of L 2  in the convey direction. However, the shape of the reference mark M according to the present invention is not limited to Z. For example, an oblique third straight line may intersect the first and second straight lines to form the reference mark M. 
   The sheet P printed with the image on its front surface is fixed by the fixing unit  70  and delivered onto the delivery tray  82  by the delivery rollers  81 . When duplex image printing is to be performed, after the image is printed and fixed on the front surface, the sheet P is conveyed downward by the guide  83  and sent to the reversal path  84 . The sheet P entering the reversal path  84  is reversed by the reversal convey rollers  85  and sent to the reversal convey path  86 . The sheet P entering the reversal convey path  86  is sent to the image printing section  60  again via the feeder  40 . 
   A method of detecting the reference mark M in printing the image on the back surface of the sheet P will be described.  FIG. 5  is a view showing an example of the method of detecting the reference mark M in printing the image on the back surface of the sheet P. As shown in  FIG. 5 , the sheet P is sent by the registration rollers  43  in the convey direction at a predetermined timing to be conveyed to the image printing section  60 . The line sensor S 2  reads and detects the reference mark M printed on the front surface of the sheet P from under the reversed sheet P. The position information of the reference mark M detected by the line sensor S 2  is output to a control section  1  shown in  FIG. 6  which has an arithmetic operating section  2 . 
   The arithmetic operating section  2  shown in  FIG. 6  obtains the shrinkage factor of the sheet P on the basis of the position information on the reference mark M in mark printing and the position information on the reference mark M detected by the line sensor S 2 . More specifically, the arithmetic operating section  2  obtains the shrinkage factor of the sheet P that has shrunk by the fixing process on the basis of the position information on the reference mark M before fixing and the position information on the reference mark M after fixing. 
   A method of calculating the shrinkage factor of the sheet P by the arithmetic operating section  2  will be described with reference to  FIG. 7 .  FIG. 7  is a view for explaining a process of obtaining the position and magnification of the image to be printed on the back surface. 
   For example, when the line sensor S 2  scans two portions on scanning lines L 4  and L 5  of the reference mark M, as shown in  FIG. 7 , it reads intersection points P 1  to P 3  where the scanning line L 4  intersects the reference mark M and intersection points P 4  to P 6  where the scanning line L 5  intersects the reference mark M. Although the line sensor S 2  scans the two portions of the reference mark M in the first embodiment, it can scan three or more portions. In this case, the line sensor S 2  reads the points where the scanning lines of respective scanning intersect the reference mark M. 
   A first length calculation unit  2 A obtains a length Y 2  between the intersection points P 1  and P 3  or between the intersection points P 4  and P 6 . The length Y 2  represents the length of the reference mark M after fixing in the main scanning direction on the front surface. The first length calculation unit  2 A also obtains a length A between the intersection points P 2  and P 3 , a length B between the intersection points P 1  and P 2 , a length C between the intersection points P 5  and P 6 , and a length D between the intersection points P 4  and P 5 . Information on the length Y 2  of the reference mark M after fixing in the main scanning direction on the front surface is output to a second length calculation unit  2 B and shrinkage factor calculation unit  2 C. Pieces of information on the lengths B and D are output to the second length calculation unit  2 B. The length Y 2  corresponds to the “first length after fixing” of the present invention. 
   The second length calculation unit  2 B obtains a length X 2  of the reference mark M in the sub-scanning direction on the basis of the convey speed of the sheet P, the main-scanning time interval required when the line sensor S 2  scans in the main scanning direction along the scanning lines L 5  and L 4 , and the lengths Y 2 , B, and D. The main-scanning time interval required when the line sensor S 2  scans in the main scanning direction along the scanning lines L 5  and L 4  is a preset time interval and stored in the storage section  3 . For example, the second length calculation unit  2 B obtains the length X 2  in the sub-scanning direction on the basis of the following equation. Note that a length E between the intersection points P 1  and P 4  satisfies:
 
 E =(convey speed [mm/sec] of sheet  P )×(main-scanning time interval [sec])
 
A relation (D−B):Y 2 =E:X 2  is established from the similarity. Expansion of this relation yields:
 
 X 2= {Y 2/( D−B )}× E    (1)
 
   The second length calculation unit  2 B calculates the length X 2  in the sub-scanning direction in accordance with the above equation. Information on the length X 2  is output to the shrinkage factor calculation unit  2 C. The length X 2  corresponds to the “second length after fixing” of the present invention. 
   The shrinkage factor calculation unit  2 C obtains the shrinkage factor of the sheet P from the lengths X 2  and Y 2  after fixing and the lengths X 1  and Y 1  of the reference mark M in mark printing which are stored in the storage section  3 . X 2 /X 1  corresponds to the shrinkage factor in the sub-scanning direction, and Y 2 /Y 1  corresponds to the shrinkage factor in the main scanning direction. Regarding the shrinkage factor in the main scanning direction, the shrinkage factor calculation unit  2 C calculates a shrinkage factor (Y 2 /Y 1 ) of the sheet P in the main scanning direction on the basis of the length Y 2  obtained by the first length calculation unit  2 A and the length Y 1  of the reference mark M in mark printing. Regarding the shrinkage factor in the sub-scanning direction, the shrinkage factor calculation unit  2 C calculates a shrinkage factor (X 2 /X 1 ) of the sheet P in the sub-scanning direction on the basis of the length X 2  obtained by the second length calculation unit  2 B and the length X 1  of the reference mark M in mark printing. 
   For example, if X 1 =10 [mm], Y 1 =10 [mm], X 2 =9.9 [mm], and Y 2 =9.9 [mm], the shrinkage factor in the sub-scanning direction is 99 [%], and the shrinkage factor in the main scanning direction is also 99 [%]. 
   A magnification determination unit  2 D determines the magnification of an image to be printed on the back surface on the basis of the shrinkage factor of the sheet P calculated by the shrinkage factor calculation unit  2 C. The magnification of the image to be printed on the back surface will be described. The magnification determination unit  2 D reduces the original image data on the image to be printed on the back surface in accordance with the shrinkage factor obtained by the shrinkage factor calculation unit  2 C. For example, if the shrinkage factor in the sub-scanning direction is 99 [%], the length in the sub-scanning direction of the image to be printed on the back surface is set to 99 [%] the original image data. If the shrinkage factor in the main scanning direction is 99 [%], the length in the main scanning direction of the image to be printed on the back surface is set to 99 [%] the original image data. Thus, the image to be printed on the back surface is reduced as a whole to 97.01 [%] the original back surface image data and printed on the back surface. 
   A position determination unit  2 E determines the position of the image printing region on the back surface on the basis of the shrinkage factor of the sheet P which is calculated by the shrinkage factor calculation unit  2 C. Regarding the position of the image printing region on the back surface, it is determined with reference to the reference point PA after fixing. For example, as shown in  FIG. 7 , assume that a position spaced apart from the reference point PA of the reference mark M after fixing by a predetermined distance is determined as the boundary of the image printing region on the back surface. The position of the reference point PA after fixing shifts from the position in mark printing by an amount corresponding to the shrinkage factor of the sheet P. Thus, the position determination unit  2 E determines the image printing region on the back surface with reference to the reference point PA after fixing which has been shifted by the shrinkage. 
   In mark printing, as shown in  FIG. 4 , the reference mark M is printed such that the reference point PA is located at a position which is at the distance a from the trailing edge of the sheet P and at the distance b from the side surface of the sheet P. After the image is transferred to the front surface and fixed, the position of the reference point PA after fixing shifts by the amount corresponding to the shrinkage factor of the sheet P to be located at a distance c from the trailing edge and at a distance d from the side surface, as shown in  FIG. 7 . For example, if the shrinkage factor in the main scanning direction is 99 [%] and the shrinkage factor in the sub-scanning direction is 99 [%], the distance c is shorter than the distance a by 1.0 [%], and the distance d is shorter than the distance d by 1.0 [%]. In this manner, the position which shifts from the position of the reference point PA in mark printing by the amount corresponding to the shrinkage factor is determined as the reference point PA after fixing. The position which is at the distance c from the trailing edge and at the distance d from the side surface is determined as the position of the reference point PA, and a position spaced apart from the reference point PA by a predetermined distance is determined as the boundary of the image printing region on the back surface. 
   The control section  1  controls the image printing section  60  on the basis of the magnification of the image to be printed on the back surface from the magnification determination unit  2 D and the position information on the image printing region on the back surface from the position determination unit  2 E. The image printing section  60  prints an image on the back surface of the sheet P under the control of the control section  1 . 
   The control section  1  is connected to the respective units of the image printing apparatus, e.g., the image reader  20 , feeder  40 , image write unit  50 , image printing section  60 , and fixing unit  70 , and controls processes such as transfer, fixing, and reversal. The control section  1  comprises a CPU or the like and reads an arithmetic operation program from a storage section (not shown) to execute the function of the arithmetic operating section  2 . 
   Although not shown, the image printing apparatus comprises an operation panel including an input unit and display unit. When a key or the like on the operation panel is pressed, a signal corresponding to the pressed key is input to the control section  1 . The display unit displays an image or a text such as a message on the window in accordance with the indication of a display signal output from the control section  1 . 
   The control operation of the image printing apparatus according to the first embodiment will be described with reference to  FIG. 8 .  FIG. 8  is a flowchart showing the control operation of the image printing apparatus according to the first embodiment of the present invention. 
   (Step S 01 ) 
   In step S 01 , the image printing section  60  prints an image and the reference mark M on the front surface of the sheet P. This will be described in detail. At a predetermined timing, the registration rollers  43  feed the sheet P conveyed from the feed tray  30  to the image printing section  60 , as shown in  FIG. 2 . When the sheet detection sensor S 1  detects the leading edge of the sheet P, the image printing section  60  prints an image at a position preset with reference to the leading edge of the sheet P, as shown in  FIG. 3 . Furthermore, the image printing section  60  prints the cutting marks K 1  to K 4 , and the substantially Z-shaped reference mark M shown in  FIG. 4  at a preset position in a region outside the cutting marks K 1  to K 4 . The reference mark M is printed at one portion on the trailing edge side of the sheet P. 
   The size and position information on the reference mark M in mark printing are stored in the storage section  3 . For example, as the size of the reference mark M, the length X 1  in the sub-scanning direction and the length Y 1  in the main scanning direction during mark printing are stored in the storage section  3 . As the position information on the reference mark M, the position information on the reference point PA is stored. For example, the distance a from the trailing edge of the sheet P and the distance b from the side surface are stored in the storage section  3  as the position information on the reference point PA. 
   (Step S 02 ) 
   In step S 02 , the sheet P is conveyed to the fixing unit  70 . The fixing unit  70  fixes the images of the cutting marks K 1  to K 4  and reference mark M on the front surface. 
   (Step S 03 ) 
   In step S 03 , the sheet P fixed with the image on its front surface is reversed and conveyed to the image printing section  60  again. More specifically, the sheet P fixed with the image on its front surface is conveyed downward by the convey path switching guide  83  shown in  FIG. 1  and sent to the reversal path  84 . The sheet P is reversed by the reversal convey rollers  85  and sent to the image printing section  60  again via the reversal convey path  86 . 
   (Step S 04 ) 
   In step S 04 , as shown in  FIG. 5 , the reversed sheet P is sent by the registration rollers  43  to the image printing section  60  at a predetermined timing. The line sensor S 2  reads the reference mark M after fixing on the front surface from under the reversed sheet P. Position information on the reference mark M read by the line sensor S 2  is output to the arithmetic operating section  2  shown in  FIG. 6 . 
   For example, when the line sensor S 2  scans two portions on the scanning lines L 4  and L 5  of the reference mark M, as shown in  FIG. 7 , it reads the points P 1  to P 3  where the scanning line L 4  intersects the reference mark M and the points P 4  to P 6  where the scanning line L 5  intersects the reference mark M. 
   (Step S 05 ) 
   In step S 05 , the first length calculation unit  2 A obtains the length Y 2  between the points P 1  and P 3 . Alternatively, the first length calculation unit  2 A can obtain the length between the points P 4  and P 6  and determine it as the length Y 2 . The length Y 2  represents the length of the reference mark M after fixing in the main scanning direction on the front surface. The first length calculation unit  2 A also obtains the length A between the points P 2  and P 3 , the length B between the points P 1  and P 2 , the length C between the points P 5  and P 6 , and the length D between the points P 4  and P 5 . Information on the length Y 2  of the reference mark M after fixing in the main scanning direction is output to the second length calculation unit  2 B and shrinkage factor calculation unit  2 C. Pieces of information on the lengths B and D are output to the second length calculation unit  2 B. 
   (Step S 06 ) 
   In step S 06 , the second length calculation unit  2 B obtains the length X 2  of the reference mark M in the sub-scanning direction on the basis of the convey speed of the sheet P, the main-scanning time interval required when the line sensor S 2  scans in the main scanning direction along the scanning lines L 5  and L 4 , and the lengths Y 2 , B, and D. The second length calculation unit  2 B obtains the length X 2  in the sub-scanning direction in accordance with the above equation (1). Information on the length X 2  is output to the shrinkage factor calculation unit  2 C. 
   (Step S 07 ) 
   In step S 07 , the shrinkage factor calculation unit  2 C obtains the shrinkage factor of the sheet P on the basis of the lengths X 2  and Y 2  obtained by the first and second length calculation units  2 A and  2 B and the lengths X 1  and Y 1  of the reference mark M in mark printing which are stored in the storage section  3 . X 2 /X 1  corresponds to the shrinkage factor in the sub-scanning direction, and Y 2 /Y 1  corresponds to the shrinkage factor in the main scanning direction. For example, if X 1 =10 [mm], Y 1 =10 [mm], X 2 =9.9 [mm], and Y 2 =9.9 [mm], the shrinkage factor in the sub-scanning direction is 99 [%], and the shrinkage factor in the main scanning direction is also 99 [%]. 
   (Step S 08 ) 
   In step S 08 , the magnification determination unit  2 D determines the magnification of the image to be printed on the back surface on the basis of the shrinkage factor obtained in step S 07 . The magnification determination unit  2 D reduces the original image data on the image to be printed on the back surface in accordance with the shrinkage factor obtained by the shrinkage factor calculation unit  2 C. For example, if the shrinkage factor in the sub-scanning direction is 99 [%], the length in the sub-scanning direction of the image data for the image to be printed on the back surface is set to 99 [%] the original image data. If the shrinkage factor in the main scanning direction is 99 [%], the length in the main scanning direction of the image data for the image to be printed on the back surface is set to 99 [%] the original image data. Thus, the image data for the image to be printed on the back surface is reduced as a whole to 97.01 [%] the original image data to print an image on the back surface on the basis of the image data. 
   (Step S 09 ) 
   In step S 09 , the position determination unit  2 E determines the position of the image to be printed on the back surface on the basis of the shrinkage factor obtained in step S 07 . The position determination unit  2 E determines the position of the image printing region on the back surface with reference to the position of the reference point PA after fixing. The position of the reference point PA after fixing on the front surface shifts by an amount corresponding to the shrinkage factor of the sheet P. Thus, the image printing region on the back surface is determined with reference to the reference point PA after fixing which has shifted. 
   In mark printing, as shown in  FIG. 4 , the reference mark M is printed such that the reference point PA is located at a position which is at the distance a from the trailing edge of the sheet P and at the distance b from the side surface of the sheet P. However, the position of the reference point PA after fixing shifts by the amount corresponding to the shrinkage factor of the sheet P to be located at the distance c from the trailing edge and at the distance d from the side surface, as shown in  FIG. 7 . For example, if the shrinkage factor in the main scanning direction is 99 [%] and the shrinkage factor in the sub-scanning direction is 99 [%], the distance c is shorter than the distance a by 1.0 [%], and the distance d is shorter than the distance d by 1.0 [%]. In this manner, the position which shifts by the amount corresponding to the shrinkage factor is determined as the reference point PA after fixing. The position which is at the distance c from the trailing edge and at the distance d from the side surface is determined as the position of the reference point PA, and a position spaced apart from the reference point PA by a predetermined distance is determined as the boundary of the image printing region on the back surface. 
   (Step S 10 ) 
   In step S 10 , the image printing section  60  prints an image on the back surface of the sheet P. At this time, an image reduced by an amount corresponding to the shrinkage factor is printed in the image printing region on the back surface determined in step S 09 . As a result, even if the sheet P shrinks due to the fixing process after the image is printed on the front surface, the image is printed on the back surface by determining its position and magnification in accordance with the shrinkage. Thus, the position and size of the image printed on the front surface can be set to coincide with the position and size of the image printed on the back surface. 
   As in the image printing apparatus according to the first embodiment, the shrinkage factor of the sheet P can be obtained by only printing the reference mark M at one portion on the sheet P. The line sensor S 2  can thus be more downsized than the prior art, and the read range of the line sensor S 2  can be narrowed more than the prior art. As the read range of the line sensor S 2  can be narrowed, the capacity of the memory to save data read by the line sensor S 2  can be decreased. 
   As the reference mark M is printed at one portion on the sheet P, it can be read by the line sensor S 2  within a short period of time. Hence, the line sensor S 2  can be set upstream of the image printing section  60 , and can read the reference mark M to obtain the magnification and position of the image to be printed on the back surface. In this manner, as the position and magnification of the image are determined on the basis of the reference mark M which is read immediately before printing the image on the back surface, the accuracies of the position and magnification can improve. 
   A blowing mechanism may be provided as a removing member for the line sensor S 2 , to remove a foreign substance such as paper dust attaching to the line sensor S 2  by blowing. 
   Second Embodiment 
   The arrangement of an image printing apparatus according to the second embodiment is the same as that of the image printing apparatus according to the first embodiment, and a description thereof will accordingly be omitted. Differences between the first and second embodiments will be described with reference to the control block diagram of the image printing apparatus according to the second embodiment shown in  FIG. 9 . 
   The image printing apparatus according to the second embodiment comprises a reference point calculation unit  2 F and third length calculation unit  2 G in place of the first and second length calculation units  2 A and  2 B provided to the image printing apparatus according to the first embodiment. Except for this, the image printing apparatus according to the second embodiment is identical to that of the first embodiment. 
   The processes performed by the reference point calculation unit  2 F and third length calculation unit  2 G will be described with reference to  FIGS. 7 ,  9 , and  10 A to  10 C.  FIGS. 10A to 10C  are views for explain a process of obtaining the position and magnification of an image to be printed on the back surface. 
   In the same manner as in the first embodiment, when a line sensor S 2  scans two portions on scanning lines L 4  and L 5  of a reference mark M, as shown in  FIGS. 7 and 10A , it reads points P 1  to P 3  where the scanning line L 4  intersects the reference mark M and points P 4  to P 6  where the scanning line L 5  intersects the reference mark M. Alternatively, the line sensor S 2  may scan three or more portions of the reference mark M. 
   As shown in  FIG. 10B , the reference point calculation unit  2 F extends an imaginary line L 6  which connects the points P 1  and P 4 , an imaginary line L 7  which connects the points P 2  and P 5 , and an imaginary line L 8  which connects the points P 3  and P 6 . The reference point calculation unit  2 F determines points where these straight lines intersect as reference points PA and PB after fixing on the front surface. 
   As shown in  FIG. 10C , the third length calculation unit  2 G extends the imaginary line L 8  which connects the points P 3  and P 6 , draws an imaginary line L 9  as a perpendicular from the reference point PA to the imaginary line L 8 , and obtains a length Y 2  from the reference point PA to an intersection point P 7  of the imaginary lines L 8  and L 9 . The third length calculation unit  2 G also obtains a length X 2  from the intersection point P 7  to the reference point PB. 
   A shrinkage factor calculation unit  2 C obtains the shrinkage factor of a sheet P from the lengths X 2  and Y 2  obtained by the third length calculation unit  2 G and lengths X 1  and Y 1  of the reference mark M in mark printing which are stored in the storage section  3 . X 2 /X 1  corresponds to the shrinkage factor in the sub-scanning direction, and Y 2 /Y 1  corresponds to the shrinkage factor in the main scanning direction. 
   For example, if X 1 =10 [mm], Y 1 =10 [mm], X 2 =9.9 [mm], and Y 2 =9.9 [mm], the shrinkage factor in the sub-scanning direction is 99 [%], and the shrinkage factor in the main scanning direction is also 99 [%]. 
   A magnification determination unit  2 D determines the magnification of image data, when an image is to be printed on the back surface, on the basis of the shrinkage factor of the sheet P calculated by the shrinkage factor calculation unit  2 C. The magnification determination unit  2 D reduces the original back surface image data in accordance with the shrinkage factor. For example, if the shrinkage factor in the sub-scanning direction is 99 [%] and the shrinkage factor in the main scanning direction is 99 [%], the lengths in the sub-scanning direction and main scanning direction are reduced to 99 [%] the lengths of the original image data. 
   In the same manner as in the first embodiment, a position determination unit  2 E determines the position of the image printing region on the back surface on the basis of the shrinkage factor of the sheet P which is calculated by the shrinkage factor calculation unit  2 C. The position determination unit  2 E determines a position shifting from the position of the reference point PA in mark printing by an amount corresponding to the shrinkage factor as the reference point PA of the image on the front surface after fixing. As shown in  FIG. 7 , a point at a distance c from the trailing edge and a distance d from the side surface is determined as the position of the reference point PA, and a position spaced apart from the reference point PA by a predetermined distance is determined as the boundary of the image printing region on the back surface. 
   The control operation of the image printing apparatus according to the second embodiment of the present invention will be described with reference to  FIG. 11 .  FIG. 11  is a flowchart showing the control operation of the image printing apparatus according to the second embodiment of the present invention. 
   (Step S 20 ) 
   In step S 20 , in the same manner as in step S 01  of the first embodiment, an image printing section  60  prints an image and the reference mark M on the front surface of the sheet P. The image printing section  60  prints an image at a position preset with reference to the leading edge of the sheet P, as shown in  FIG. 3 . Furthermore, the image printing section  60  prints cutting marks K 1  to K 4 , and the substantially Z-shaped reference mark M shown in  FIG. 4  at a preset position in a region outside the cutting marks K 1  to K 4 . 
   A storage section  3  stores the size and position information on the reference mark M in mark printing in the same manner as in the first embodiment. For example, as the size of the reference mark M, the length X 1  in the sub-scanning direction and the length Y 1  in the main scanning direction during mark printing are stored in the storage section  3 . As the position information on the reference mark M, the position information on the reference point PA is stored. For example, a distance a from the trailing edge of the sheet P and a distance b from the side surface are stored in the storage section  3  as the position information on the reference point PA. 
   (Step S 21 ) 
   In step S 21 , the sheet P is conveyed to a fixing unit  70 , in the same manner as in step S 02  of the first embodiment. The fixing unit  70  fixes the images of the cutting marks K 1  to K 4  and reference mark M on the front surface. 
   (Step S 22 ) 
   In step S 22 , the sheet P fixed with the image on its front surface is reversed and conveyed to the image printing section  60  again, in the same manner as in step S 03  of the first embodiment. 
   (Step S 23 ) 
   In step S 23 , as shown in  FIG. 5 , at a predetermined timing, registration rollers  43  feed the reversed sheet P to the image printing section  60 , in the same manner as in step S 04  of the first embodiment. The line sensor S 2  reads the reference mark M after fixing on the front surface from under the reversed sheet P. Position information on the reference mark M read by the line sensor S 2  is output to a control section  1  having an arithmetic operating section  4  shown in  FIG. 9 . 
   For example, when the line sensor S 2  scans two portions on the scanning lines L 4  and L 5  of the reference mark M, as shown in  FIGS. 7 and 10A , it reads the points P 1  to P 3  where the scanning line L 4  intersects the reference mark M and the points P 4  to P 6  where the scanning line L 5  intersects the reference mark M, to detect the reference mark M. 
   (Step S 24 ) 
   In step S 24 , the reference point calculation unit  2 F obtains the reference point of the reference mark M after fixing on the front surface. For example, as shown in  FIG. 10B , the reference point calculation unit  2 F extends the imaginary line L 6  which connects the points P 1  and P 4 , the imaginary line L 7  which connects the points P 2  and P 5 , and the imaginary line L 8  which connects the points P 3  and P 6 . The reference point calculation unit  2 F determines points where these straight lines intersect as the reference points PA and PB after fixing on the front surface. 
   (Step S 25 ) 
   In step S 25 , as shown in  FIG. 10C , the third length calculation unit  2 G extends the imaginary line L 8  which connects the points P 3  and P 6 , draws the imaginary line L 9  as the perpendicular from the reference point PA to the imaginary line L 8 , and obtains the length Y 2  from the reference point PA to the intersection point P 7  and the length X 2  from the intersection point P 7  to the reference point PB. 
   (Step S 26 ) 
   In step S 26 , the shrinkage factor calculation unit  2 C obtains the shrinkage factor of the sheet P on the basis of the lengths X 2  and Y 2  obtained by the third length calculation unit  2 G and the lengths X 1  and Y 1  of the reference mark M in mark printing which are stored in the storage section  3 . X 2 /X 1  corresponds to the shrinkage factor in the sub-scanning direction, and Y 2 /Y 1  corresponds to the shrinkage factor in the main scanning direction. For example, if X 1 =10 [mm], Y 1 =10 [mm], X 2 =9.9 [mm], and Y 2 =9.9 [mm], the shrinkage factor in the sub-scanning direction is 99 [%], and the shrinkage factor in the main scanning direction is also 99 [%]. 
   (Step S 27 ) 
   In step S 27 , the magnification determination unit  2 D determines the magnification of the image to be printed on the back surface on the basis of the shrinkage factor obtained in step S 26 . In the same manner as in step S 08  of the first embodiment, the magnification determination unit  2 D reduces the original back surface image data in accordance with the shrinkage factor obtained by the shrinkage factor calculation unit  2 C. For example, if the shrinkage factor in the sub-scanning direction is 99 [%] and the shrinkage factor in the main scanning direction is 99 [%], the lengths in the sub-scanning direction and main scanning direction are reduced to 99 [%] the lengths of the original image data. 
   (Step S 28 ) 
   In step S 28 , in the same manner as in step S 09  of the first embodiment, the position determination unit  2 E determines the position of the image to be printed on the back surface on the basis of the shrinkage factor obtained in step S 26 . The position determination unit  2 E determines a position shifted from the position of the reference point PA in mark printing by an amount corresponding to the shrinkage factor as the reference point PA of the image on the front surface after fixing. As shown in  FIG. 7 , the position determination unit  2 E determines a position at the distance c from the trailing edge and the distance d from the side surface as the reference point PA, and a position spaced apart from the reference point PA by a predetermined distance as the boundary of the image printing region on the back surface. 
   (Step S 29 ) 
   In step S 29 , the image printing section  60  prints an image on the back surface of the sheet P, in the same manner as in step S 10  of the first embodiment. At this time, an image reduced by an amount corresponding to the shrinkage factor is printed in the image printing region on the back surface determined in step S 28 . As a result, even if the sheet P shrinks due to the fixing process after the image is printed on the front surface, the image is printed on the back surface by determining its the position and magnification in accordance with the shrinkage. Thus, the position and size of the image printed on the front surface can be set to coincide with the position and size of the image printed on the back surface. 
   In the same manner as in the image printing apparatus according to the first embodiment, the line sensor S 2  can be downsized more than the prior art, and the read range of the line sensor S 2  can be narrowed more than the prior art. Also, the capacity of the memory to save data read by the line sensor S 2  can be decreased. Since the reference mark M is printed at one portion on the sheet P in the same manner as in the image printing apparatus according to the first embodiment, it can be read by the line sensor S 2  within a short period of time. Hence, the reference mark M can be read immediately before printing an image on the back surface, so the accuracies of the position and magnification can be improved. 
   Furthermore, in the same manner as in the image printing apparatus according to the first embodiment, a blowing mechanism may be provided as a removing member for the line sensor S 2 , to remove a foreign substance such as paper dust attaching to the line sensor S 2  by blowing air or the like.