Abstract:
There is described a duplex image-forming apparatus having a function of forming images onto both sides of a transfer material. The apparatus includes an image-forming section to respectively form an obverse-side image and a reverse-side image on a photoreceptor element by scanning a light beam, modulated with image signals based on pixel-clock signals and reflected from a polygon mirror rotating at an operating velocity based on polygon-clock signals; a transferring section to transfer said obverse-side image and said reverse-side image onto both surfaces of said recording sheet; a fixing section to fix the images onto both surfaces of said recording sheet; and a clock-frequency changing section to change a pixel-clock frequency, and a polygon-clock frequency, corresponding to a degree of shrinkage of said recording sheet caused by a fixing operation performed in said fixing section, at a transition time of an image-forming operation from one side to another side.

Description:
BACKGROUND OF THE INVENTION 
   The present invention relates to an image-forming apparatus and specifically relates to a duplex image-forming apparatus having a function of forming images onto both sides of a transfer material (a recording sheet). 
   In an image-forming apparatus, such as a laser printer, etc., a printing operation is performed through the processes of exposing, developing, transferring and fixing. Concretely speaking, initially, a latent image to be printed is formed on a surface of a photoreceptor drum by irradiating a light beam (a laser beam), which is modulated on the basis of the image data, onto the photoreceptor drum, and then, the developing device develops the latent image with toner. The developed toner image is transferred onto the transfer material (the recording sheet), and then, the fixing device fixes the toner image onto the transfer material. As a result, the image is formed on the transfer material. 
   When fixing the toner image onto the transfer material, the transfer material tends to shrink, since the fixing heat dehydrates the transfer material during the fixing operation. Specifically, when performing the duplex printing, the shrinkage of the transfer material, generated during the fixing operation for the obverse surface of the transfer material, causes a misregistration error between obverse and reverse images, since the obverse image size becomes different from the reverse image size. To cope with this problem, it has been possible to adjust the magnification factor in a sheet feeding direction by changing the processing velocity between the obverse image forming operation and the reverse image forming operation. 
   It has been a problem, however, that the abovementioned method, for adjusting the magnification factor by changing the processing velocity, cannot be applied for adjusting the magnification factor in a width direction of the transfer material, though it is applicable for adjusting the magnification factor in a sheet feeding direction (a lateral direction of the transfer material). Especially in a color image-forming apparatus having a plurality of image-forming units each of which includes an exposure unit, a photoreceptor drum, a developing device, etc., even if an image-forming operation, performed in one of the plurality of image-forming units, is completed, sometimes, another image-forming operation, performed in another one of the plurality of image-forming units, still continues, due to gaps arranged between the exposure units (process gaps). Therefore, the change of the processing velocity, before the image-forming operations in all of the plurality of image-forming units are completed, would result in a color misregistration error. To avoid the color misregistration error, the processing velocity should be changed after the image-forming operations in all of the plurality of image-forming units are completed. Accordingly, it has been a problem that the total printing time is getting long, resulting in a reduction of the print productivity. 
   SUMMARY OF THE INVENTION 
   To overcome the abovementioned drawbacks in conventional image-forming apparatus, it is an object of the present invention to provide a duplex image-forming apparatus, which makes it possible to accurately adjust the obverse and reverse image sizes without lowering the print productivity. 
   Accordingly, to overcome the cited shortcomings, the abovementioned object of the present invention can be attained by duplex image-forming apparatus described as follow.
     (1) An apparatus for forming a duplex image on a recording sheet, the duplex image includes an obverse-side image formed on an obverse surface of the recording sheet and a reverse-side image formed on a reverse surface of the recording sheet, the apparatus comprising: an image-forming section to respectively form the obverse-side image and the reverse-side image on a photoreceptor element by scanning a light beam, modulated with image signals based on pixel-clock signals and reflected from a polygon mirror rotating at an operating velocity based on polygon-clock signals, onto the photoreceptor element; a transferring section to transfer the obverse-side image and the reverse-side image formed on the photoreceptor element onto the obverse surface and the reverse surface of the recording sheet, respectively; a fixing section to fix the obverse-side image and the reverse-side image onto the obverse surface and the reverse surface of the recording sheet, respectively; and a clock-frequency changing section to change a pixel-clock frequency, being a frequency of the pixel-clock signals, and a polygon-clock frequency, being a frequency of the polygon-clock signals, corresponding to a degree of shrinkage of the recording sheet caused by a fixing operation performed in the fixing section, at a transition time of an image-forming operation in the image-forming section from the obverse-side image to the reverse-side image, and vice versa.   (2) The apparatus of item 1, wherein the clock-frequency changing section determines a pixel-clock frequency changing-rate and a polygon-clock frequency changing-rate, based on a first shrinkage rate of the recording sheet in a paper conveying direction.   (3) The apparatus of item 2, wherein the clock-frequency changing section further determines the pixel-clock frequency changing-rate, based on both the first shrinkage rate of the recording sheet in a paper conveying direction and a second shrinkage rate of the recording sheet in a main-scanning direction.   (4) An apparatus for forming a duplex color image on a recording sheet, the duplex color image includes an obverse-side color image formed on an obverse surface of the recording sheet and a reverse-side color image formed on a reverse surface of the recording sheet, the apparatus comprising: a plurality of image-creating units, each of which corresponds to each of a plurality of unicolor images and forms each of the plurality of unicolor images by scanning a light beam, modulated with image signals based on pixel-clock signals and reflected from a polygon mirror rotating at an operating velocity based on polygon-clock signals, to respectively form the obverse-side color image and the reverse-side color image on a photoreceptor element; a transferring section to transfer the obverse-side color image and the reverse-side color image formed on the photoreceptor element onto the obverse surface and the reverse surface of the recording sheet, respectively; a fixing section to fix the obverse-side color image and the reverse-side color image onto the obverse surface and the reverse surface of the recording sheet, respectively; and a clock-frequency changing section to change a pixel-clock frequency, being a frequency of the pixel-clock signals for each of the plurality of image-creating units, and a polygon-clock frequency, being a frequency of the polygon-clock signals for each of the plurality of image-creating units, corresponding to a degree of shrinkage of the recording sheet caused by a fixing operation performed in the fixing section, at a transition time of an image-forming operation from the obverse-side color image to the reverse-side color image, and vice versa.   (5) The apparatus of item 4, wherein the clock-frequency changing section determines a pixel-clock frequency changing-rate and a polygon-clock frequency changing-rate, based on a first shrinkage rate of the recording sheet in a paper conveying direction.   (6) The apparatus of item 5, wherein the clock-frequency changing section further determines the pixel-clock frequency changing-rate, based on both the first shrinkage rate of the recording sheet in a paper conveying direction and a second shrinkage rate of the recording sheet in a main-scanning direction.   (7) The apparatus of item 5, further comprising: a setting section to set timings for forming the plurality of unicolor images separately between a first phase for forming the obverse-side color image and a second phase for forming the reverse-side color image, based on the pixel-clock frequency changing-rate and the polygon-clock frequency changing-rate determined by the clock-frequency changing section.   (8) The apparatus of item 4, wherein the clock-frequency changing section changes the pixel-clock frequency and the polygon-clock frequency separately for every one of the plurality of image-creating units.   (9) The apparatus of item 4, wherein each of polygon-clock signals, corresponding to each of the plurality of image-creating units, is generated from each of a plurality of clock generating sources being independent from each other.   (10) The apparatus of item 4, further comprising: a multi-clock generating section to generate a plurality of clock signals, each of which is independently employed for each of the plurality of image-creating units, from a clock signal outputted by a single clock generating source; wherein each of polygon-clock signals, corresponding to each of the plurality of image-creating units, is generated from each of the plurality of clock signals generated by the multi-clock generating section.   (11) The apparatus of item 4, further comprising: a reference signal detecting section to detect main-scanning reference signals in main-scanning paths of the light beam; and a phase controlling section to control a phase of the polygon-clock signals, based on a phase difference between a first main-scanning reference signal of a reference color and a second main-scanning reference signal of another color, so as to determine the phase of the polygon-clock signals corresponding to the second main-scanning reference signal of the other color, each of the first and second main-scanning reference signals being one of the main-scanning reference signals detected by the reference signal detecting section.   (12) The apparatus of item 11, wherein the phase controlling section independently performs a first operation for controlling the phase of the polygon-clock signals employed for forming the obverse-side color image on the obverse surface of the recording sheet, and a second operation for controlling the phase of the polygon-clock signals employed for forming the reverse-side color image on the reverse surface of the recording sheet; and wherein the phase controlling section changes the first operation to the second operation every time when the first operation for each of the plurality of unicolor images is completed, or vise versa.   (13) The apparatus of item 11, wherein the phase controlling section independently performs a first operation for controlling the phase of the polygon-clock signals employed for forming the obverse-side color image on the obverse surface of the recording sheet, and a second operation for controlling the phase of the polygon-clock signals employed for forming the reverse-side color image on the, reverse surface of the recording sheet; and wherein the phase controlling section changes the first operation to the second operation when the first operation for all of the plurality of unicolor images is completed, and vice versa.   

   Further, to overcome the abovementioned problems, other image forming apparatus, embodied in the present invention, will be described as follow:
     (14) An image forming apparatus, characterized in that,
       in the image forming apparatus, which is provided with image forming means for scanning a light beam on a photoreceptor element using a polygon mirror to form an image, transfer means for transferring onto recording paper the image formed by this image forming means, and fixing means for fixing the aforementioned transferred image onto this recording paper, and has a duplex printing function,   the image forming apparatus is further provided with a change means for changing the frequency of pixel clock signals for driving the aforementioned light beam and the frequency of polygon clock signals for driving the aforementioned polygon mirror, based on the degree of the shrinkage of the recording paper caused by the fixing means in the process of shifting from image formation on one side by the aforementioned image forming means to image formation on the other side by the image forming means.   
       (15) An image forming apparatus, characterized in that,
       in the image forming apparatus, which is provided with image forming means further containing multi-color image creating units for image formation, the aforementioned image forming means being a device for scanning a light beam on a photoreceptor element using a polygon mirror for each of these image creating units to form an image, transfer means for transferring onto recording paper the image formed by the aforementioned multi-color image creating units and fixing means for fixing the aforementioned transferred image onto this recording paper, and has a duplex printing function,   the image forming apparatus is further provided with a change means for changing the frequency of pixel clock signals for driving the aforementioned light beam of the aforementioned image creating units and the frequency of polygon clock signals for driving the aforementioned polygon mirror, based on the degree of the shrinkage of the recording paper caused by the fixing means in the process of shifting from image formation on one side by the aforementioned image forming means to image formation on the other side by the image forming means.   
       (16) The image forming apparatus, described in item 14 or 15, characterized in that the aforementioned change means determines the change rate of the frequency of the aforementioned pixel clock signals and the frequency of polygon clock signals based on the shrinkage rate of the recording paper in the feed direction.   (17) The image forming apparatus, described in item 16, characterized in that the aforementioned change means determines the change rate of the aforementioned pixel clock signals based on the shrinkage rates of the recording paper in the feed direction and in the main scanning direction of the recording paper.   

   The aforesaid invention allows frequencies of the pixel clock signal and polygon clock signal to be changed when image formation on one side of the recording paper is shifted to that on the other side, thereby ensuring excellent matching between the image size of the front side and that of the back side.
     (18) The image forming apparatus, described in item 16 or 17, characterized in that the image forming apparatus is further provided with a setting means for setting the timings of forming images of various colors separately between front surface image formation and back surface image formation, based on the change rate determined by the aforementioned change means.   

   The aforementioned invention allows the timings of forming images of various colors to be set separately between formation of an image on the front surface and that on the back surface. This prevents color misregistration from occurring in the traveling direction of paper.
     (19) The image forming apparatus, described in anyone of items 15-18, characterized in that the aforementioned change means changes the frequency of pixel clock signals and the frequency of polygon clock signals for each of the aforementioned multi-color image creating units.   

   The aforementioned invention allows the frequency of pixel clock signals and that of polygon clock signals to be changed for each of the aforementioned multi-color image creating units, whereby printing time is reduced.
     (20) The image forming apparatus, described in anyone of items 15-19, characterized in that each of polygon clock signals corresponding to each of the aforementioned image creating units is generated by each of clock signal sources being different from each other.   

   The aforementioned invention provides easy generation of a polygon clock signal corresponding to each of the image creating units.
     (21) The image forming apparatus, described in anyone of items 15-19, characterized in that the image forming apparatus is further provided with clock generation means that separately generates clock signals used in each of the image creating units, from the clock signals produced from one clock signal source, and polygon clock signals corresponding to the image creating units are each produced from clock signals generated from the aforementioned clock generation means.   

   The above invention provides high-precision correction of color misregistration, since polygon clock signals corresponding to the image creating units are each produced from one clock generation means.
     (22) The image forming apparatus, described in anyone of items 15-19, characterized in that the image forming apparatus is further provided with detecting means for detecting the main scanning reference signal in the light beam in the aforementioned multi-color image creating units, and phase control means for controlling the phase of this polygon clock signal, based on the phase difference between the main scanning reference signal detected by the aforementioned detecting means and the polygon clock signal.   (23) The image forming apparatus, described in items 22, characterized in that the aforementioned phase control means provides phase control of the polygon clock signal corresponding to image formation on the front surface of the recording paper, and that on the back surface of the paper, where these two types of control are provided independently of each other, and, upon completion of phase control of one side of the recording paper, phase control of the other side of the paper is started for each color.   (24) The image forming apparatus, described in items 23, characterized in that, upon completion of entire phase control corresponding to image formation on one side of the recording paper, the aforementioned phase control means is switched over to phase control corresponding to that on the other-side of the recording paper, for each color.   

   The invention described above provides polygon signal phase control, thereby enabling correction of color misregistration of not more than one pixel in the paper feed direction. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Other objects and advantages of the present invention will become apparent upon reading the following detailed description and upon reference to the drawings in which: 
       FIG. 1  is a cross sectional view representing the image forming apparatus GH according to the present invention; 
       FIG. 2  is a block diagram representing the configuration of the control circuit of an exposure unit ( 1 Y,  1 M,  1 C and  1 K); 
     FIGS.  3 ( a ) and  3 ( b ) are drawings representing the shrinkage of transfer material (recording paper) due to fixing operation; 
       FIG. 4  is a block diagram representing a polygon drive clock generation circuit  105   a;    
       FIG. 5  is a clock diagram representing a polygon drive clock generation circuit  105   b;    
       FIG. 6  is a clock diagram representing a polygon drive clock generation circuit  105   c  with consideration given to polygon drive clock signal phase control; 
       FIG. 7  is a clock diagram representing a polygon drive clock generation circuit  105   d  with consideration given to polygon drive clock signal phase control; 
       FIG. 8  is a timing chart representing timings of image formation on the front and back surfaces for each color; and 
       FIG. 9  is a timing chart representing timings of image formation on the front and back surfaces for each color. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
   Referring to the drawings, an image-forming apparatus, embodied in the present invention, will be detailed in the following. 
     FIG. 1  shows a cross-sectional view of an image forming apparatus GH, such as a color copier, etc., embodied in the present invention. As shown in  FIG. 1 , the image forming apparatus GH comprises a plurality of image forming units  10 Y,  10 M,  10 C,  10 K, a transferring section  20 , a fixing section  30 , serving as a fixing means, and a paper conveying section  40  including a paper re-feeding mechanism (an ADU mechanism). 
   The image forming unit  10 Y, for forming a toner image of yellow (Y) color, is provided with an exposure unit  1 Y, a photoreceptor drum  2 Y serving as an image-forming element, developing device  3 Y, a photoreceptor cleaning device  4 Y. In the exposure unit  1 Y, a polygon mirror (not shown in the drawings) scans a light beam (a laser beam) onto the photoreceptor drum  2 Y under the controlling actions embodied in the present invention (described later, referring to FIGS.  2 - 9 ). A scanning operation of the light beam irradiated onto photoreceptor drum  2 Y forms a latent image on the surface of photoreceptor drum  2 Y. The developing device  3 Y develops the latent image formed on the photoreceptor drum  2 Y with toner of Y (yellow) color. 
   The image forming section  10 M, for forming a toner image of magenta (M) color, is provided with a exposure unit  1 M, a photoreceptor drum  2 M serving as an image-forming element, a developing device  3 M, a photoreceptor cleaning device  4 M. An image forming section  10 C, for forming a toner image of cyan (C) color, is provided with the exposure unit  1 C, a photoreceptor drum  2 C serving as an image-forming element, a developing device  3 C, a photoreceptor cleaning device  4 C. An image forming section  10 K, for forming a toner image of black (K) color, is provided with an exposure unit  1 K, a photoreceptor drum  2 K serving as an image-forming element, a developing device  3 K, a photoreceptor cleaning device  4 K. Accordingly, in each of image forming sections  10 M,  10 C,  10 K, the image-forming operation is performed through the same processes as those in image-forming section  10 Y. 
   In the transferring section  20 , each of the Y, M, C, K color toner images, formed in each of the image-forming sections  10 Y,  10 M,  10 C,  10 K, is sequentially transferred onto an intermediate transferring belt  20 B, rotating along photoreceptor drums  2 Y,  2 M,  2 C,  2 K, by means of primary transferring rollers  20 Y,  20 M,  20 C,  20 K, so as to form a full-color toner image by synthesizing the Y, M, C, K color toner images (the primary transferring operation). When a recording paper P is conveyed to a position of a secondary transferring roller  20 S, the full-color toner image formed on the intermediate transferring belt  20 B is transferred onto an obverse side of the recording paper P at a time by means of the secondary transferring roller  20 S equipped in the transferring section  20  (the secondary transferring operation). 
   The full-color toner image transferred onto the recording paper P is fixed onto the recording paper P by the heat-fixing processing performed in the fixing section  30 . 
   A paper conveying section  40  comprises a paper-circulating path  40 A, a reverse conveying path  40 B and a paper re-feeding section  40 C. When forming a reverse image, the recording-paper P ejected from the fixing section  30  is passed through the paper-circulating path  40 A, and then, when the recording-paper P arrives at the reverse conveying path  40 B, the rotating direction of the rollers, equipped in the reverse conveying path  40 B, is reversed so as to convey the recording-paper P to paper the re-feeding section  40 C. Accordingly, at the time when the recording-paper P passes through paper the re-feeding section  40 C, the obverse side, on which the full-color toner image is already fixed, faces upward. The recording paper P passed through the paper re-feeding section  40 C is reversed in its obverse and reverse sides by paper the feeding roller  50 , and is conveyed again by the secondary transferring roller  20 S so as to transfer another full-color toner image onto another side (a reverse side) of the recording-paper P at a time. 
   Next, referring to FIG.  2 - FIG. 7 , a configuration of a controlling system for each of the exposure units  1 Y,  1 M,  1 C,  1 K will be detailed in the following. Although a controlling system of the exposure unit  1  is exemplified as a single unit in the following explanation referring to  FIG. 2  in order to simplify the explanation, the following explanation will be applied for each of the exposure units  1 Y,  1 M,  1 C,  1 K. 
     FIG. 2  shows a block diagram of the configuration of the controlling circuit for the exposure unit  1 . As shown in  FIG. 2 , the controlling circuit for the exposure unit  1  comprises a CPU  101 , a crystal oscillator  102 ,  104 , a pixel clock generation circuit  103 , a polygon drive clock generation circuit  105 , a horizontal synchronizing circuit  106  and a PWM (Pulse Width Modulation) signal generation circuit  107 . Further, the exposure unit  1  comprises an index sensor  11 , a LD (Laser Diode) drive unit  12  and a polygon motor  13 . 
   A CPU  101  provides various control operations according to the control program for image forming apparatus stored in a memory (not illustrated). 
   To put it more specifically, the CPU  101  changes the setting of the frequency value for the polygon drive clock signal (hereinafter referred to as “polygon drive clock signal frequency”) in the creation of image on the back side, based on the degree of shrinkage of recording paper caused by fixing operation during printing on the front surface, in order to adjust the scale in paper feed direction during the creation of image on the back side. In this case, the CPU  101  separately changes the polygon drive clock frequency for each color, where the rate of change in the polygon drive clock frequency for each color remains the same. The CPU  101  sends to the polygon drive clock generation circuit the control signal for instructing generation of the polygon drive clock signal having an updated polygon drive clock frequency. 
   When the polygon drive clock frequency value has been increased by a change in polygon drive clock frequency, the polygon mirror speed is increased to raise the speed for main scanning of the laser beam on the photoreceptor drum by means of a polygon mirror (main scanning speed). In other words, the size of formed image is reduced if the main scanning speed is increased when process speed is constant, and is increased if the main scanning speed is reduced. 
   When the main scanning speed is changed by changing the polygon drive clock frequency described above, the scale in the direction of main scanning is also changed. Then in order to adjust the scale in the direction of main scanning, the CPU  101  changes the setting of the value for pixel clock signal frequency (hereinafter referred to as “pixel clock frequency”) in image creation on the back side. Since shrinkage of the transfer material (recording paper) due to fixing operation also occurs in the direction of main scanning, the CPU  101  changes the settings of pixel clock frequency, including the resealing in the direction of main scanning due to shrinkage of the transfer material. In this case, the CPU  101  changes the pixel clock frequency of each color separately, where the rate of change in the pixel clock frequency for each color remains the same. The CPU  101  sends to the pixel clock generation circuit  103  the control signal for instructing generation of the pixel clock signal having an updated pixel clock frequency. 
   In this case, the following alternative configurations are also possible: (1) The degree of the shrinkage of the transfer material P is instructed manually by an operator from the control panel (not illustrated). (2) A document reader is used to provide automatic detection by reading the pattern for measuring the size of the image formed on the front and back sides of the transfer material. (3) Automatic detection is provided by the means for detecting the size of the image formed on the front and back sides of the transfer material inside the paper feed path. 
   As illustrated in  FIG. 3 , let assume, for example, that recording paper P before undergoing fixing operation has a longitudinal length (in paper feed direction) of L mm and a width (in the direction of main scanning) of W mm ( FIG. 3  (a)), and the sizes are shrunken to L′ mm and W′ mm ( FIG. 3  (b)) due to fixing on the surface of the recording paper P. Also assume that polygon drive clock frequency during image creation on the front side is F 0 , and that during image creation on the back side is F. Then we get F=(L/L′) F 0 . Let us further assume that pixel clock frequency during image creation on the front side is f 0  and that during image creation on the back side is f. Then we get: f=(L/L′) (W/W′) f 0 . 
   The CPU  101  changes the setting of polygon drive clock frequency separately for each color, where the rate of change (L/L′) in the polygon drive clock frequency for each color remains the same. In the same manner, the CPU  101  changes the setting of pixel clock frequency separately for each color, where the rate of change (L/L′) (W/W′) in the pixel clock frequency for each color remains the same. 
   The CPU  101  adjusts the timing of image formation for each color by changing the color misregistration correction value corresponding to the correction value for inter-process gaps in response to the change in polygon drive clock frequency. This is because the number of lines corresponding to the inter-process gap is changed by adjusting the scale in the paper feed direction (sub-scanning direction) without changing the process speed. The inter-process gap is stored in the memory (not illustrated) as a color misregistration correction value. This color misregistration correction value is set in conformity to the formation of image on the front surface. When the image is formed on the back side by adjusting the scale in the sub-scanning direction, this color misregistration correction value must be changed in response to shrinkage rate (rate of change) in the sub-scanning direction; otherwise, color misregistration will occur. This makes it possible to use a different color misregistration correction value in the image creation on the front side from that in the image creation on the back side. 
   In  FIG. 2 , a crystal oscillator  102  generates a reference clock signal having a predetermined frequency and sends it to the pixel clock generation circuit  103 . 
   In response to the control signal received from the CPU  101 , the pixel clock generation circuit  103  generates a pixel clock signal for driving laser beam in the exposure unit  1 , from the reference clock signal sent from the crystal oscillator  102 . 
   A crystal oscillator  104  generates the reference clock signal having a predetermined frequency and sends it to a polygon drive clock generation circuit  105 . 
   In response to the control signal received from the CPU  101 , the polygon drive clock generation circuit  105  generates from the reference clock signal received from the crystal oscillator  104  the polygon drive clock signal for driving the polygon mirror for applying laser beam to a photoreceptor drum. The details of the polygon drive clock generation circuit  105  are described with reference to  FIGS. 4 through 7 . 
   A horizontal synchronizing circuit  106  synchronizes the pixel clock signal generated by the pixel clock generation circuit  103  with an index signal (described later) detected by an index sensor  11 , and sends it to a PWM signal generation circuit. 
   In response to the pixel clock signal received from the horizontal synchronizing circuit  106 , the PWM signal generation circuit  107  generates the PWM signal corresponding to image data and sends it to a LD drive unit  12 . 
   The index sensor  11  in the exposure unit  1  uses an index mirror (not illustrated) to detect the main scanning reference signal (index signal) of laser beam irradiated from the polygon mirror and sends it to the horizontal synchronizing circuit  106 . 
   Based on the PWM signal produced from the PWM signal generation circuit  107 , a LD drive unit  12  generates the drive signal for controlling the LD. 
   The polygon M (motor)  13  is a DC brush-less motor for controlling LD irradiation according to the drive signal produced from the LD drive unit  12 . It rotates and drives the polygon mirror in response to the polygon clock signal produced from the polygon drive clock generation circuit  105 . 
   The following describes the polygon drive clock generation circuit  105  of FIG.  2 . 
     FIG. 4  shows the internal configuration of a polygon drive clock generation circuit  105   a  as an example of the polygon drive clock generation circuit  105  according to the present invention. 
   As shown in  FIG. 4 , the polygon drive clock generation circuit  105  comprises crystal oscillators  104 Y,  104 M,  104 C and  104 K, and frequency divider circuits  108 Y,  108 M,  108 C and  108 K. 
   The CPU  101  shown in  FIG. 2  separately sets the division ratio of each of the frequency divider circuits  108 Y,  108 M,  108 C and  108 K, and sends to each of these frequency divider circuits the control signal for instructing frequency division at the preset division ratio. Especially, the CPU  101  changes the setting of division ratio in each frequency divider circuit in order to change the polygon drive clock frequency for each color for the purpose of adjusting the scale in the paper feed direction during backside image creation when creating an image on the back side in duplex printing mode. 
   In response to the control signal produced from the CPU  101 , the frequency divider circuit  108 Y generates the yellow polygon clock signal by dividing the frequency of the reference clock signal produced from the crystal oscillator  104 Y, and sends it to a polygon M in an exposure unit  1 Y. 
   In response to the control signal produced from the CPU  101 , the frequency divider circuit  108 M generates the magenta polygon clock signal by dividing the frequency of the reference clock signal produced from the crystal oscillator  104 M, and sends it to the polygon M in an exposure unit  1 M. 
   In response to the control signal produced from the CPU  101 , the frequency divider circuit  108 C generates the cyan polygon clock signal by dividing the frequency of the reference clock signal produced from the crystal oscillator  104 C, and sends it to the polygon M in an exposure unit  1 C. 
   In response to the control signal produced from the CPU  101 , the frequency divider circuit  108 K generates the black polygon clock signal by dividing the frequency of the reference clock signal produced from the crystal oscillator  104 M, and sends it to the polygon M in an exposure unit  1 M. 
     FIG. 5  shows the internal configuration of a polygon drive clock generation circuit  105   b  as an example of the polygon drive clock generation circuit  105  according to the present invention. 
   As shown in  FIG. 5 , the polygon drive clock generation circuit  105   b  comprises a crystal oscillator  104 , PLLs (phase Locked Loops)  109 Y,  109 M,  109 C and  109 K, and frequency divider circuits  110 Y,  110 M,  110 C and  110 K. 
   In  FIG. 5 , the CPU  101  separately sets the frequency converted value at the PLLs  109 Y,  109 M,  109 C and  109 K, and products the control signals for instructing frequency conversion to send them to each of these PLLs separately. Especially, the CPU  101  changes the setting of frequency conversion values of each PLL in order to change the polygon drive clock frequency for each color for the purpose of adjusting the scale in the paper feed direction during backside image creation when creating an image on the back side in duplex printing mode. 
   In response to the control signal produced from the CPU  101 , the PLL  109 Y converts the frequency of the reference clock signal produced from the crystal oscillator  104  and sends the frequency-converted signal to the frequency divider circuit  110 Y. 
   In response to the control signal produced from the CPU  101 , the PLL  109 M converts the frequency of the reference clock signal produced from the crystal oscillator  104  and sends the frequency-converted signal to the frequency divider circuit  110 M. 
   In response to the control signal produced from the CPU  101 , the PLL  109 C converts the frequency of the reference clock signal produced from the crystal oscillator  104  and sends the frequency-converted signal to the frequency divider circuit  110 C. 
   In response to the control signal produced from the CPU  101 , the PLL  109 K converts the frequency of the reference clock signal produced from the crystal oscillator  104  and sends the frequency-converted signal to the frequency divider circuit  110 K. 
   The frequency divider circuit  110 Y divides the frequency of the clock signal-sent from the PLL  109 Y at a predetermined division ratio to generate the yellow polygon clock signal and sends it to the polygon M in the exposure unit  1 Y. 
   The frequency divider circuit  110 M divides the frequency of the clock signal sent from the PLL  109 M at a predetermined division ratio to generate the magenta polygon clock signal and sends it to the polygon M in the exposure unit  1 M. 
   The frequency divider circuit  110 C divides the frequency of the clock signal sent from the PLL  109 C at a predetermined division ratio to generate the cyan polygon clock signal and sends it to the polygon M in the exposure unit  1 C. 
   The frequency divider circuit  110 K divides the frequency of the clock signal sent from the PLL  109 K at a predetermined division ratio to generate the black polygon clock signal and sends it to the polygon M in the exposure unit  1 K. 
     FIG. 6  shows the internal configuration of a polygon drive clock generation circuit  105   c  as an example of the polygon drive clock generation circuit  105  according to the present invention. 
   As shown in  FIG. 6 , the polygon drive clock generation circuit  105   c  comprises a crystal oscillator  104   h  for creating an image on the front side, a crystal oscillator  104   r  for creating an image on the back side, frequency divider circuits  111   h  and  111   r , phase control circuits  112   h  and  112   r , and selectors  113 Y,  113 M,  1113 C and  113 K. 
   In  FIG. 6 , the CPU  101  changes the settings of the division ratio in the frequency divider circuits  111   h  and  111   r  in order to change the polygon drive clock frequency for each color for the purpose of adjusting the scale in the paper feed direction during backside image creation when creating an image on the back side in duplex printing mode. The CPU  101  then sends the control signal for instructing frequency division at the preset division ration to these frequency divider circuits. Further, the CPU  101  sends to the phase control circuits  112   h  and  112   r  the control signal for instructing phase control of the clock signal that has been frequency-divided by the frequency divider circuit&#39;s  111   h  and  11 , respectively. Further, the CPU  101  sends to the selectors  113 Y,  113 M,  113 C and  113 K the selection signal for selecting between the clock signal produced from the phase control circuit  112   h  for creating an image on the front side or the clock signal produced from the phase control circuit  112   r  for creating an image on the back side. 
   The frequency divider circuit  111   h  divides the frequency of the reference clock signal produced from the crystal oscillator  104   h  at the division ratio set by the CPU  101 , and sends it to the phase control circuit  112   h.    
   The frequency divider circuit  111   r  divides the frequency of the reference clock signal produced from the crystal oscillator  104   r  at the division ratio set by the CPU  101 , and sends it to the phase control circuit  112   r.    
   Phase control circuit  112   h  detects the phase difference between the index signal of the reference color, for instance, a black color, and the other index signal of another color, for instance, yellow color, among the index signals of the colors (Yellow Index and others) detected by index sensor  11 . Then, phase control circuit  112   h  conducts phase control of the polygon clock signal to determine a phase of a polygon drive clock for another color, for example, for yellow against a polygon drive clock for a color representing a reference, for example, for a black color, based on the phase difference, and outputs controlled clock signal to a selector for the corresponding color. 
   Phase control circuit  112   r  detects the phase difference between the index signal of the reference color, for instance, a black color, and the other index signal of another color, for instance, yellow color, among the index signals of the colors (Yellow Index and others) detected by index sensor  11 . Then, phase control circuit  112   h  conducts phase control of the polygon clock signal to determine a phase of a polygon drive clock for another color, for example, for yellow against a polygon drive clock for a color representing a reference, for example, for a black color, based on the phase difference, and outputs controlled clock signal to a selector for the corresponding color. 
   In response to the selection signal produced from the CPU  101 , the selector  113 Y selects either one of the clock signals produced from two phase control circuits  112   h  and  112   r , and sends it to the polygon M in the exposure unit  1 Y as a yellow polygon clock signal. 
   In response to the selection signal produced from the CPU  101 , the selector  113 M selects either one of the clock signals produced from two phase control circuits  112   h  and  112   r , and sends it to the polygon M in the exposure unit  1 M as a magenta polygon clock signal. 
   In response to the selection signal produced from the CPU  101 , the selector  113 C selects either one of the clock signals produced from two phase control circuits  112   h  and  112   r , and sends it to the polygon M in the exposure unit  1 C as a cyan polygon clock signal. 
   In response to the selection signal produced from the CPU  101 , the selector  113 K selects either one of the clock signals produced from two phase control circuits  112   h  and  112   r , and sends it to the polygon M in the exposure unit  1 K as a black polygon clock signal. 
     FIG. 7  shows the internal configuration of a polygon drive clock generation circuit  105   d  as an example of the polygon drive clock generation circuit  105  according to the present invention. 
   As shown in  FIG. 7 , the polygon drive clock generation circuit  105   d  comprises a crystal oscillator  104   h  for creating an image on the front side, a crystal oscillator  104   r  for creating an image on the back side, a selector  114 , a frequency divider circuit  115 , and a phase control circuit  116 . 
   In  FIG. 7 , the CPU  101  sends to the selector  114  the selection signal for selecting between the reference clock signal produced from the crystal oscillator  104   h  for creating an image on the front side and that produced from the crystal oscillator  104   r  for creating an image on the back side. The CPU  101  then changes the settings of the division ratio in the frequency divider circuit  115  in order to change the polygon drive clock frequency for each color for the purpose of adjusting the scale in the paper feed direction during backside image creation when creating an image on the back side in duplex printing mode. Then it sends the control signal for instructing frequency division at the preset division ration to the frequency divider circuit  115 . Further, the CPU  101  sends to the phase control circuit  116  the control signal for instructing phase control of the clock signal that has been frequency-divided by the frequency divider circuit  115 . 
   In response to the selection signal produced from the CPU  101 , the selector  114  selects either one of the reference clock signals produced from two oscillators  104  and  104   r , and sends it to the frequency divider circuit  115 . 
   In response to the control signal produced from the CPU  101 , the frequency divider circuit  115  divides the frequency of the reference clock signal produced from the crystal oscillator  104   h  or  104   r  at the preset division ratio, and sends it to the phase control circuit  116 . 
   Phase control circuit  116  detects the phase difference between the index signal of the reference color, for instance, a black color, and the other index signal of another color, for instance, yellow color, among the index signals of the colors (Yellow Index and others) detected by index sensor  11 . Then, phase control circuit  112   h  conducts phase control of the polygon clock signal to determine a phase of a polygon drive clock for another color, for example, for yellow against a polygon drive clock for a color representing a reference, for example, for a black color, based on the phase difference, and outputs controlled clock signal to a selector for the corresponding color. 
   Two signal sources for image formation on the front and back sides (crystal oscillator  104   h  and  104   r ) are arranged in FIG.  7 . It is also possible to make arrangements in such a way that the frequency of the reference clock signal produced from one signal source is converted by two PLLs, and is divided by a frequency divider circuit, as shown in FIG.  5 . 
   The following shows the operation of the present Embodiment: 
   First, the following describes the operation in the duplex printing mode where the polygon drive clock generation circuits  105   a ,  105   b  and  105   c  (hereinafter referred to as “ 105 ”) is applied. It should be noted that the following explanation of the operation refers to the case where the recording paper P is shrunken by the step of fixing on the front side, as shown in FIG.  3 . 
   When the duplex printing mode for a document g placed on the document tray of an automatic document feed apparatus  201  is specified by the operation of a key or touch panel on the image forming apparatus GH, the document g is fed by a feed means, and images on both sides of the document g are scanned and exposed by the optical system of a document image scanning/exposure apparatus  202 . It is then read into a line image sensor CCD. 
   The read image is subjected to photoelectric conversion through the line image sensor CCD. The analog signal undergoing photoelectric conversion through the line image sensor CCD is decomposed into each of the colors Y, M, C and K by an image processor (not illustrated), and is stored into an image memory (not illustrated) as image data. 
   Based on the image clock signal produced from a horizontal synchronizing circuit, the PWM signal generation circuit  107  generates the PWM signal for driving the LD in response to the Y-color image data stored in the image memory. 
   Based on the PWM signal generated in a PWM signal generation circuit  107 , the LD drive unit  12  in the exposure unit  1 Y generates the drive signal for emitting a laser beam. According to the Y-color polygon clock signal generated by the polygon drive clock generation circuit  105 , the polygon mirror of a polygon M 13  is rotated, and the laser beam driven by the aforementioned drive signal is emitted to a photoreceptor drum  2 Y for scanning. 
   A Y-color electrostatic latent image is formed on the photoreceptor drum  2 Y by the scanning of laser beam emitted through the polygon mirror. The electrostatic latent image on the photoreceptor drum  2 Y is developed by Y-color toner supplied from the developing device  3 Y. 
   The same steps are taken in the exposure units  1 M,  1 C and  1 K. Electrostatic latent images for magenta, cyan and black colors are formed on the photoreceptor drums  2 M,  2 C and  2 K. These electrostatic latent images on the photoreceptor drums  2 M,  2 C and  2 K are developed by toner of magenta, cyan and black colors. 
   Then a magenta toner image formed by an image forming unit  10 M is transferred on a rotating intermediate transferring belt  20 B and is superimposed on the yellow toner image that has already been transferred. Upon completion of magenta color transfer, the toner remaining on the peripheral surface of the photoreceptor drum  2 M after transfer is removed by an image forming device cleaning means  4 M. Then the next image formation cycle (image formation cycle for back side) is started. 
   Then a cyan toner image formed by the image forming unit  10 C is transferred on the rotating intermediate transferring belt  20 B and is superimposed on the yellow and magenta toner images that have already been transferred. Upon completion of cyan color transfer, the toner remaining on the peripheral surface of the photoreceptor drum  2 C after transfer is removed by an image forming device cleaning means  4 C. Then the next image formation cycle (image formation cycle for back side) is started. 
   Then a black toner image formed by the image forming unit  10 K is transferred on the rotating intermediate transferring belt  20 B and is superimposed on the yellow, magenta and cyan toner images that have already been transferred. Upon completion of the primary black color transfer, the toner remaining on the peripheral surface of the photoreceptor drum  2 K after transfer is removed by an image forming device cleaning means  4 K. Then the next image formation cycle (image formation cycle for back side) is started. 
   When the CPU  101  has determined that the image has been created on the front side by the image forming unit  10 Y, the value obtained by multiplying the yellow polygon drive clock, frequency in the front side image creation mode by L/L′ is set as a yellow polygon dive clock frequency in the back side image creation mode. Then the frequency control signal is sent to the polygon dive clock generation circuit  105 . In response to the frequency control signal sent from the CPU  101 , the polygon drive clock generation circuit  105  generates the yellow polygon clock signal for back side image creation, and this signal is sent to the polygon M 13  in the exposure unit  1 Y. 
   When polygon drive CLK generating circuit  105   c  shown in  FIG. 6  is applied, polygon drive CLK of Y color controlled in terms of a phase is outputted based on the phase difference of the index signal between reference color K and Y color, on phase control circuit  112   r  for the reverse side use, and polygon drive CLK of the phase control circuit  112   r  is selected in selector section  113 Y. Namely, polygon CLK signals for Y color, whose frequency and phase have been adjusted for the image forming operation on the reverse surface of the recording medium, are generated on polygon drive CLK generating circuit  105   c.    
   In addition to the change in the setting of the yellow polygon drive frequency, the value obtained by multiplying the yellow pixel clock frequency in the front side image creation mode by (L/L′). (W/W′) is set as a yellow pixel clock frequency in the back side image creation mode, and the frequency control signal is sent to the pixel clock generation circuit  103 . 
   Based on the pixel cock signal with the frequency changed, the PWM signal generation circuit  107  generates the PWM signal in conformity to the yellow image data for back side stored in the image memory. The LD drive unit  12  in the exposure unit  1 Y emits the laser beam based on the PWM signal generated by the PWM signal generation circuit  107 . The polygon M 13  rotates the polygon mirror based on the yellow polygon clock signal for back side image creation, and the aforementioned laser beam is emitted to the photoconductor  2 Y for scanning. 
   Upon completion of image creation on the front side by the image forming units  10 M,  10 C and  10 K, the settings of the polygon drive clock frequency and pixel clock frequency are changed for image creation on the back side, the image creation on the back side is started, and image creation on the back side is started, similarly to the case of image forming unit  10 Y. 
   As described above,  FIG. 8  shows the timing chart when the polygon drive clock generation circuits  105   a , 105   b  and  105   c  given in  FIGS. 4 through 6  is applied. As shown in the timing chart of  FIG. 8 , frequency change and phase control of the polygon clock signals for various colors are carried out at the timed intervals α, β, γ and δ, respectively. 
   When the polygon drive clock generation circuit  105   d  of  FIG. 7  is applied, phase control for back side is selected upon completion of the phase control of the polygon clock signal for front side image creation for all colors. Accordingly, timing for image formation on the front and back sides is determined as follows: The frequency and phase of the polygon clock signal and pixel clock frequency are changed at the timed interval ε upon completion of image creation on the front side for all colors (yellow, magenta, cyan and black), as shown in the timing chart of FIG.  9 . Then the image creation on the back side is started. 
   As described above, for during back sided image creation in the duplex printing mode, the image forming apparatus GH as the present Embodiment changes the frequency of the polygon clock signal to adjust the scale in the paper feed direction (sub-scanning direction), and the frequency of the pixel clock signal to adjust the scale in the main scanning direction, thereby ensuring high precision registration between the front and back. 
   Further, the frequency of the polygon clock signal and frequency of the pixel clock signal are changed for each of the image creation units (image forming units  10 Y,  10 M,  10 M and  10 K) for various colors. As shown in the timing chart of  FIG. 8 , each image creation unit changes the polygon drive clock frequency and pixel clock frequency at the timed intervals α, β, γ and δ, respectively, upon completion of image creation on the front side, and then shifts to the step of image creation on the back side, thereby reducing the printing time. 
   The polygon drive clock generation circuit&#39;s  105   c  and  105   d  shown in  FIGS. 6 and 7  provides phase control of the polygon clock signal based on the index signal for each color. This allows the write timing control of exposure units  1 Y,  1 M,  1 C and  1 K, and permits correction of color misregistration of not more than one pixel in the paper feed direction (sub-scanning direction), thereby enduring a high-precision color registration. 
   It should be noted that the present Embodiment is not restricted to the above description. Variations are possible as appropriate, without departing from the spirit of the present invention. 
   The present invention provides the following effects: 
   (1) The present invention allows frequencies of the pixel clock signal and polygon clock signal to be changed when image formation on one side of the recording paper is shifted to that on the other side, thereby ensuring excellent matching between the image size of the front side and that of the back side. 
   (2) The present invention allows the timings of forming images of various colors to be set separately between formation of an image on the front surface and that on the back surface. This prevents color misregistration from occurring in the traveling direction of paper. 
   (3) The present invention allows the frequency of pixel clock signals and that of polygon clock signals to be changed for each of the multi-color image creating units, whereby printing time is reduced. 
   (4) The present invention provides easy generation of a polygon clock signal corresponding to each of the image creating units. 
   (5) The present invention provides precision-precision correction of color misregistration since polygon clock signals corresponding to the image creating units are produced from one clock generation means. 
   (6) The prevent invention provides polygon signal phase control, thereby enabling correction of color misregistration of not more than one pixel in the paper feed direction. 
   Disclosed embodiment can be varied by a skilled person without departing from the spirit and scope of the invention.