Patent Publication Number: US-7221893-B2

Title: Image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
   This application claims priority under 35 USC 119 from Japanese Patent Application No. 2004-278045, the disclosure of which is incorporated by reference herein. 
   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to an image forming apparatus, and more particularly to an image forming apparatus using a continuous paper transported as a recording medium and which is designed so as to prevent a color-to-color misregistration state which tends to be caused due to a variation in the transport speed of the continuous paper when respective color toner images are transferred onto the continuous paper. 
   2. Description of the Related Art 
   In a tandem type color laser printer, photoreceptor drums carrying toner images of respective colors (cyan, magenta, yellow, black, and so forth) are provided along a transport path for a recording medium. Each of the photoreceptor drums is arranged such that after a surface thereof is charged by a charger, a surface potential is decreased by means of an exposure device so that a latent image is formed thereon. The latent image is developed into a toner image by means of a development device. Then, the toner image carried by each photoreceptor drum is transferred onto the recording medium by means of a transfer device. 
   Among such tandem type color laser printers is one which uses a continuous paper as a recording medium. 
   As will be understood, it is required that toner images on respective photoreceptor drums be sequentially transferred in precise registration onto the continuous paper, thereby preventing a color-to-color misregistration state of the toner images. In order to satisfy such a requirement, in a tandem type color laser printer using a continuous paper, it has so far been proposed to apply a tension to the continuous paper so that the continuous paper is tensioned before and after the transfer, thereby preventing a color-to-color misregistration state of toner images from being caused because of the continuous paper going slack or getting wrinkled (for example, refer to JP-A No. 6-64244). 
   However, it has sometimes happened that a color-to-color misregistration state is caused despite application of a tension to the continuous paper The inventor has conducted an analysis of color-to-color misregstration states, and found out causes for such states as mentioned below. 
   When a toner image on a photoreceptor drum is transferred onto a continuous paper, since that portion of the photoreceptor drum&#39;s surface where no toner image is formed is charged, the continuous paper is electrostatically attracted to the photoreceptor drum. Furthermore, since a coverage of the toner image (a proportion of area occupied by the toner image) differs from one photoreceptor drum to another, the electrostatic force of attraction of the continuous paper also differs from one photoreceptor drum to another. 
   For this reason, a tension applied to the continuous paper between adjacent ones of the photoreceptor drums is varied so that tensions applied to the continuous paper between the respective photoreceptor drums become nonuniform. Consequently, the transport speed of the continuous paper is varied, and the variation in the transport speed of the continuous paper causes a color-to-color misregistration state in a transfer process. 
   SUMMARY OF THE INVENTION 
   The present invention has been made with a view to solving the above-described problems, and provides an apparatus that enables toner images to be transferred in precise registration onto a continuous paper. 
   According to an aspect of the present invention, an image forming apparatus includes a print section including plural print units arranged along a transport path for a continuous paper for sequentially transferring respective color toner images in registration onto the continuous paper, wherein each of the plural print units includes a rotatable image carrier device, a charger device for charging a surface of the image carrier device at a predetermined surface potential, an exposure device for exposing the image carrier device so as to decrease the surface potential thereby causing a latent image to be formed on the image carrier device, a development device for developing the latent image on the image carrier device into a toner image, and a transfer device disposed to face the image carrier device across the transport path for the continuous paper therefrom for causing the toner image on the image carrier device to be transferred onto the continuous paper; a continuous paper transport section provided on an upstream side of a transport direction of the continuous paper for transporting the continuous paper toward the print section; a fixing section provided on a downstream side of the transport direction of the continuous paper for fixing unfixed toner images onto the continuous paper, the unfixed toner images being formed at the print section and transferred onto the continuous paper; a tensioning mechanism for imparting a predetermined tension to the continuous paper having the toner images fixed thereon; and a control unit for preventing a color-to-color misregistration state which tends to occur when respective color toner images are sequentially transferred to be overlaid onto the continuous paper. 
   Other objects, features and advantages of the present invention will become to a person of ordinary skill in the art from the following description taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Embodiments of the present invention will be described in detail based on the following figures, wherein: 
       FIG. 1  is a schematic view showing a color laser printer according to an embodiment of the present invention. 
       FIG. 2  is a schematic perspective view showing the color laser printer of  FIG. 1 . 
       FIG. 3A  shows a wrap device for use in the color laser printer according to an embodiment of the present invention, wherein a small amount of wrap is provided. 
       FIG. 3B  shows a wrap device for use in the color laser printer according to an embodiment of the present invention, wherein an increased amount of wrap is provided. 
       FIG. 4  is a block diagram illustrating a color laser printer. 
       FIG. 5  is a view useful for explaining about electrostatic forces of attraction and tensions which are provided when the coverages of toner images on all photoreceptor drums have a reference value. 
       FIG. 6  is a view useful for explaining about an electrostatic force of attraction and a tension which are provide when the coverage of a toner image on the cyan photoreceptor drum alone has a value greater than a reference value. 
       FIG. 7  is a flow chart illustrating a control for correcting a write-in timing. 
       FIG. 8  is a graph showing a relationship between coverage and electrostatic force of attraction of a continuous paper to a photoreceptor drum. 
       FIG. 9  is a graph showing a relationship between electrostatic force of attraction of a continuous paper to a photoreceptor drum and color-to-color misregistration quantity. 
       FIG. 10  is a graph showing a relationship between color-to-color misregistration quantity and write-in timing. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   As shown in  FIG. 1 , a color laser printer  10  serving as an image forming device performs image formation by an electrophotographic process. The color laser printer  10  includes a control portion  101  for controlling the whole of the color laser printer  10 . On an exterior package of the color laser printer  10  is provided an operation panel  103  for providing various information displays and carrying out various operations of the color laser printer  10 . 
   Also, as shown in  FIG. 1 , the color laser printer  10  includes a print section  30  for transferring toner images of respective colors such as yellow (Y), magenta (M), cyan (C) and black (K) in the named order to a continuous paper P and superimposing the toner images upon each other. The print portion  30  includes print units  30 Y,  30 M,  30 C and  30 K for the respective colors such as yellow (Y), magenta (M), cyan (C) and black (K), which are arranged in this order along a transport passageway from upstream to downstream in the transport direction. Hereinafter, when a distinction among YMCK is required, a corresponding one of the suffixes Y, M, C, and K will be added to the reference numeral for convenience of explanation, while when such a distinction is not required, the suffixes will be omitted. 
   A paper transport section  20  for transporting continuous paper P to the print section  30  is provided at the upstream side of the print section  30  as viewed in the transport direction. Further, at the downstream side of the print section as viewed in the transport direction, a fixing section  400  is provided which operates to fix to the continuous paper P, the unfixed full-color toner images which are formed at the respective print units  30 Y–K and transferred to the continuous paper P. 
   As shown in  FIG. 2 , the paper transport section  20  includes a main drive roller  102  around a part of which the continuous paper P is wrapped. An idle roller  104  is disposed in contact with the main drive roller  102  in such a manner as to pinch the continuous paper P at a nip portion between the main drive roller  102  and the idle roller  104 , thereby permitting the continuous paper P to be transported. Further, the main drive roller  102  is normally driven at a constant speed by means of a main drive motor  106 , and this speed constitutes a reference for the transport speed of the continuous paper P transported through the apparatus. 
   Furthermore, as shown in  FIG. 1 , the print units  30 Y–K respectively include photoreceptor drums  12 Y–K around which are provided chargers  22 Y–K, LED heads  14 Y–K, developing units  34 Y–K, transfer rollers  24 Y–K, cleaning units  28 Y–K, and neutralization chargers (not shown) in the named order as viewed in the rotational direction of the photoreceptor drums. 
   As shown in  FIG. 4 , the photoreceptor drums  12 Y–K are rotated by photoreceptor drum driving motors  31 Y–K each including a stepping motor, and the photoreceptor drum driving motors  31 Y–K are controlled in terms of rotation speed by a pulse inputted from the control unit  101  thereto. It is arranged such that each of the photoreceptor drums  12 Y–K is driven by a respective one of the photoreceptor drum driving motors  31 Y–K so that the photoreceptor drums  12 Y–K can be rotated at different speeds. 
   As shown in  FIG. 1 , the chargers  22 Y–K cause the surfaces of the respective photoreceptor drums  12 Y–K to be charged at a predetermined surface potential. The LED heads  14 Y–K line-expose the surfaces of the photoreceptor drums  12 Y–K, and thus decrease the surface potentials thereof, thereby forming latent images. Meanwhile, as shown in  FIG. 4 , the respective LED heads  14 Y–K are individually controlled by the control unit  101  so that the exposure timings (write-in timings) of the respective LED heads  14 Y–K are individually controlled by the control unit  101 . Further, the exposure device may use other components than the LED heads  14 . For example, the exposure device may be an optical scanning device arranged to perform exposure by scanning laser light. 
   In this manner, as shown in  FIG. 1 , the developing units  34 Y–K cause toners to be adhered onto the latent images formed on the photoreceptor drums  12 Y–K, thereby forming toner images. 
   The transfer rollers  24 Y–K are disposed in contact with the top surfaces of the photoreceptor drums  12 Y–K with the continuous paper P held therebetween, thereby transporting the continuous paper P in cooperation with the photoreceptor drums  12 Y–K. During the holding and transporting operation, the transfer rollers  24 Y–K are applied with a transfer bias voltage by a transfer power source  125  (see  FIG. 4 ), so that the latent images formed on the photoreceptor drums  12 Y–K are transferred onto the continuous paper P. Meanwhile, as shown in  FIG. 4 , the transfer power source  125  is controlled by the control unit  101 , and thus the transfer bias voltage applied to the transfer rollers  24 Y–K is controlled. In this regard, it is possible that different transfer bias voltages may be applied to the transfer rollers  24 Y–K respectively. 
     FIG. 3  shows a wrap device  200  for enabling the continuous paper P to be wrapped around a part of the photoreceptor drum  12 . The wrap device  200  includes two stabilizer rollers  40  which are disposed on the downstream and upstream sides of the transfer roller  24  as viewed in the transport direction. The continuous paper P is wrapped around a part of the circumferential surface of the photoreceptor drum  12  near the transfer roller, thereby stabilizing the transfer of the toner image on the photoreceptor drum  12  onto the continuous paper P. 
   Each of the stabilizer rollers  40  is rotatably attached to an end of an arm  240  which is biased at the other end by means of a spring  242  so that each stabilizer roller  40  is pressed against the photoreceptor drum  12 . Each arm  240  has an elongated aperture  240 B formed at a center portion thereof and another elongated aperture  240 A at the other end. A shaft  246  attached to a support member (not shown) is inserted through the elongated aperture  240 A formed at the other end of the arm  240 . 
   Another shaft  244  is inserted through the elongated aperture  240 B formed in the center portion of the arm  240 . The shaft  244  is formed on a rotary plate  248  attached to a rotary shaft of an arm driving motor  241  which is a stepping motor. 
   Thus, as shown in  FIG. 3B , when the rotary plate  248  is rotated by the arm driving motor  241 , the pair of arms  240  are displaced in directions S 1  and S 2  respectively so that the stabilizer rollers  40  are moved away from each other. Consequently, the amount of the continuous paper P that is wrapped around a part of the circumferential surface of the photoreceptor drum  12  is increased. As shown in  FIG. 4 , the angle of rotation of each of the arm driving motors  241 Y–K, i.e., the wrapping amount, is individually controlled by the control unit  101 . 
   The wrap device that partially wraps each photoreceptor drum  12  in the continuous paper P may be a mechanism other than the wrapping mechanism  200 , which may be any mechanism capable of causing the continuous paper P to be wrapped around a part of the photoreceptor drum  12  and adjusting the wrapping amount. 
   As shown in  FIG. 1 , non-transferred residual toners remaining on the surfaces of the photoreceptor drums  12 Y–K without being transferred to the continuous paper P are removed by the cleaning units  28 Y–K. 
   The continuous paper P having toner images transferred thereto is transported to the fixing section  400  so that the toner images of the continuous paper P are heated and melted by a flash fixing device  108  so as to be fixed onto the continuous paper P. The continuous paper P, which has been subjected to the fixing process, is transported to a tensioning mechanism  250 . 
   As shown in  FIG. 2 , the tensioning mechanism  250  includes idle rollers  132  and  130  about which the continuous paper P is entrained. 
   A tensioning roller  112  is provided between the idle rollers  132  and  130 . The tensioning roller  112  is supported at each of its axially opposite ends by one end of each of arms  114 . A shaft  116  extend through the other ends of the arms  114  so that the arms  114  are rocked about the shaft  116 . The arms  114  are biased toward the continuous paper P side by springs  120  so that the tensioning roller  112  is biased toward the continuous paper P. In this manner, a tension is imparted to the continuous paper P. 
   Further, a sub-drive roller  110  and an idle roller  128  are provided at the downstream side of the idle roller  130 . The sub-drive roller  110  is driven by a sub-drive motor  122  so that the continuous paper P is transported with the aid of the sub-drive roller  110  and idle roller  128  while being held therebetween. Meanwhile, as shown in  FIG. 4 , the sub-drive motor  122  is controlled in terms of rotation speed by the control unit  101 . 
   Further, as shown in  FIG. 2 , the rotation angle of the arms  114  is detected by an encoder  118  mounted on an end of the shaft  116 , and the result of detection is passed to the control unit  101  (see  FIG. 1 ). The control unit  101  controls the rotation speed of the sub-drive motor  122  so that the arms  114  are located at a predetermined position at all times. That is, the rotation speed of the sub-drive motor  122  is determined from a tension of the continuous paper P which is detected based on the position of the tensioning roller  112 . 
   More specifically, as the tension of the continuous paper P is increased, the amount of sag of the continuous paper P is decreased so that the tension roller  112  is pushed in a direction such that the continuous paper P does not sag (rightward as viewed in the drawing). As a result of this, the arms  114  are also rotated rightward as viewed in the drawing, and a signal indicative of a plus direction (the tension being strong) is detected at the encoder  118 . 
   By providing a feedback corresponding to this signal to the sub-drive motor  122  to thereby decrease the rotation speed of the sub-drive roller  110 , the transport speed of the continuous paper P is decreased, and consequently the amount of sag of the continuous paper P is increased so that the tensioning roller  112  is rotated in a direction such as to push in the continuous paper P (leftward as viewed in the drawing). 
   On the other hand, as the tension of the continuous paper P is decreased, the amount of sag of the continuous paper P is increased so that the tensioning roller  112  is drawn in a direction such that the continuous paper P is caused to sag (leftward as viewed in the drawing). Thus, the arms  114  are also rotated leftward as viewed in the drawing and a signal indicative of a minus direction (the tension being weak) is detected at the encoder  118 . 
   By providing a feedback corresponding to this signal to the sub-drive motor  122  to thereby increase the rotation speed of the sub-drive roller  110 , the transport speed of the continuous paper P is increased, and consequently the amount of sag of the continuous paper P is decreased so that the tensioning roller  112  is rotated in a direction such as to be pushed by the continuous paper P (rightward as viewed in the drawing). 
   Through repetition of the above-mentioned process, the sub-drive motor  110  is operated, while changing its rotation speed, to cause the continuous paper P to be transported in such a manner that the tensioning roller  112  assumes a fixed position, i.e., the tension of the continuous paper P is kept constant. 
   The tensioning mechanism for imparting a predetermined tension to the continuous paper P may be any suitable mechanism other than the tensioning mechanism  250 . 
   The continuous paper P is ejected out of the apparatus from the tensioning mechanism  250 . Description will now be made of the printing operation of the color laser printer  10 . 
   Formation of a toner image onto the photoreceptor drum  12  will first be described. The charger  22  causes the surface of the photoreceptor drum  12  to be uniformly charged at a predetermined surface potential. Subsequently, exposure is provided by the LED head  14 , and the surface potential at the exposed position of the photoreceptor drum  12  is decreased, thus resulting in an electrostatic latent image being formed on the surface of the photoreceptor drum  12 . Then, the electrostatic latent image is developed by the developing unit  34  into a toner image. 
   In the color laser printer  10  which is designed to perform multi-color full-color image formation, an adjustment is made of a timing that is set by taking into account of differences in relative position among the print units  30 Y–K, e.g., of an exposure timing with which each of the LED heads  14 Y–K provides exposure to form a latent image, and a toner image forming step similar to that described above is carried out at each of the photoreceptor drums  12 Y–K. 
   Then, the control unit  101  operates such that a transfer bias voltage is applied to each of the transfer rollers  24 Y–K with a predetermined timing and toner images formed on the photoreceptor drums  12 Y–K in respective colors such as yellow, magenta, cyan, and black are sequentially transferred onto the continuous paper P in a superposed relationship with each other, thereby forming a full-color toner image on the continuous paper P. 
   When the continuous paper P is transported until the head of the area where the full-color unfixed toner image formed on the continuous paper P reaches an entrance to an infrared ray irradiation area of the flash fixing device  108 , the control unit  101  causes flash lamps to irradiate infrared rays. Thus, when passing through the infrared ray irradiation area of the flash fixing device  108 , the unfixed toners on the continuous paper P are melted by being heated by infrared rays irradiated from the flash lamps, and solidified to be fixed on the continuous paper P after having passed through the infrared ray irradiation area. 
   The operation of this embodiment will next be described. 
   The tensioning mechanism  250  imparts a predetermined tension to the continuous paper P, as mentioned above. Further, tensions applied to the continuous paper P between the respective photoreceptor drums  12 Y–K, between the photoreceptor drum  12 Y and the idle roller  150 , and between the photoreceptor drum  12 K and the idle roller  152  as shown in  FIG. 5  are equal, and the transport speed of the continuous paper P is also kept constant. In contrast thereto, if such tensions are unequal, the transport speed is not kept constant. 
   When a toner image on the photoreceptor drum  12  is transferred onto the continuous paper P, that portion of the photoreceptor drum  12  on which no toner image is formed is charged at a given surface potential so that the continuous paper P is electrostatically attracted to the surface of the photoreceptor drum  12 . 
   However, the coverage of a toner image (the proportion of the area occupied by the toner image) differs from one image formed to another, and also among the respective photoreceptor drums  12 Y–K. Accordingly, the electrostatic force of attraction with which the continuous paper P is attracted to the surface of the photoreceptor drum  12  also differs among the respective photoreceptor drums  12 Y–K. Thus, the transporting force with which the continuous paper P is transported while being held between the respective photoreceptor drums  12 Y–K and the respective transfer rollers  24 Y–K also differs among the photoreceptor drums  12 Y–K. Consequently, tensions which the tensioning mechanism  250  applies to the continuous paper P between adjacent ones of the photoreceptor drums  12 Y–K, between the photoreceptor drum  12 Y and the idle roller  150  and between the photoreceptor drum  12 K and the idle roller  152  are varied and become unequal as shown in  FIG. 6 . For this reason, the transport speed of the continuous paper P is varied. Disadvantageously, this variation in the transport speed of the continuous paper P results in a color-to-color misregistration state that respective color toner images are out of registration. Stated differently, differences in coverage among color toner images formed on the photoreceptor drums  12 Y–K cause a color-to-color misregistration state. 
   Therefore, the control unit  101  controls one or more of the following items, based on the coverage of each of respective color toner images formed on the respective photoreceptor drums  12 Y–K, thereby preventing a color-to-color misregistration state:
     (1) the exposure timing of each of the LED heads  14 Y–K;   (2) the rotation speed of each of the drum driving motors  31 Y–K, or the rotation speed of each of the photoreceptor drums  12 Y–K);   (3) the transfer bias voltage applied to each of the transfer rollers  24 Y–K; and   (4) the rotation angle of each of the arm driving motors  41 Y–K, or the amount of wrap of the continuous paper P due to the stabilizer rollers  40 Y–K.   

   What has been mentioned above will be described below more specifically. While the photoreceptor drum  12 C will be explained below by way of example, the following explanation will also apply to each of the other photoreceptor drums  12 Y,  12 M, and  12 K. 
     FIG. 5  illustrates a case where all the coverages are at a predetermined reference level, wherein it is assumed that a reference electrostatic force of attraction with which the continuous paper P is electrostatically attracted to the photoreceptor drums  12 Y–K is 2 (the numeral enclosed by a double square in the drawing). Further, assuming that a total of the tensions applied to the whole of the continuous paper P is 10, reference tensions applied to the continuous paper P are equally apportioned by 2 each (the numerals enclosed by double square in the drawing). 
   Let it be assumed that the coverage of a toner image formed on the photoreceptor drum  12 C becomes greater than a reference value so that the electrostatic force of attraction is changed from 1 to 2 as shown in  FIG. 6 . Because of the electrostatic force of attraction being decreased, the holding and transporting force produced by the photoreceptor drum  12 C and transfer roller  24 C is decreased. Consequently, the tension applied to the continuous paper P is decreased by 0.5 from 6 to 5.5 at the upstream side of the transfer, and increased by 0.5 from 4 to 4.5 at the downstream side. 
   As a result, the continuous paper P is subjected to a force that draws it toward the upstream side, so that the transport speed of the continuous paper P is decreased. Consequently, it becomes such that a position where a toner image on the photoreceptor drum  12 C is transferred to the continuous paper P occurs earlier, thus resulting in an out-of-superposition state, or a color-to-color misregistration state of toner images. 
   Therefore, the control unit  101  performs one or more the following procedures (1) to (4), thereby making an exact superposition of toner images on the continuous paper P. 
   (1) The exposure timing of the LED head  14 C is delayed, and thus the forming position of a toner image to be formed on the photoreceptor drum  12 C is delayed, to an extent corresponding to a decrease in the speed of the continuous paper P, thereby correcting the timing with which toner images are superposed in a transfer process. 
   (2) The rotation speed of the photoreceptor drum  12 C is delayed to an extent corresponding to a decrease in the speed of the continuous paper P, thereby correcting the timing with which toner images are superposed in a transfer process. 
   (3) When the electrostatic force of attraction with which the continuous paper P is electrostatically attracted to the photoreceptor drum  12 C is small, the transfer bias voltage applied to the transfer roller  24 Y is increased so as to increase the electrostatic force of attraction with which the continuous paper P is electrostatically attracted to the photoreceptor drum  12 C, thereby keeping constant the transporting force of the continuous paper P as well as the transport speed of the continuous paper P. 
   (4) When the electrostatic force of attraction with which the continuous paper P is electrostatically attracted to the photoreceptor drum  12 C is small, the stabilizer roller  40 C is displaced so as to increase the amount of wrap, thereby keeping constant the transporting force of the continuous paper P as well as the transport speed of the continuous paper P. 
   Meanwhile, when the coverage of a toner image is small and the electrostatic force of attraction is great, the control unit  101  performs a control reverse to the above control. 
   The control by the control unit  101  will now be described specifically with respect to a case where the forming position of a toner image to be formed on the photoreceptor drum  12  is corrected by exposure timing of the LED head  14  (the above-described procedure (1)). 
     FIG. 7  is a flow chart illustrating the control by the control unit  101 .  FIG. 8  illustrates a relationship between the coverage and the electrostatic force of attraction of the continuous paper P to the photoreceptor drum  12 .  FIG. 9  shows a relationship between he electrostatic force of attraction of the continuous paper P to the photoreceptor drum  12  and the color-to-color misregistration quantity due to an increase/decrease in the speed of the continuous paper P.  FIG. 10  illustrates a relationship between the color-to-color misregistration quantity and the write-in timing of the LED head  14 C. Data shown in  FIGS. 8 to 10  are prestored in the control unit  101 . 
   Image data outputted at step  300  is inputted to the control unit  101  (see  FIG. 1 ). At step  302 , the respective color coverages, or the coverages of respective color toner images to be formed on the respective photoreceptor drums  12 Y–K are calculated. At step  304 , the electrostatic forces of attraction of the continuous paper P to the respective photoreceptor drums  12 Y–K are calculated from the calculated coverages and the data shown in  FIG. 8 . At step  306 , the color-to-color misregistration quantities at the respective photoreceptor drums  12 Y–K are calculated from the calculated electrostatic forces of attraction and the data shown in  FIG. 9 . At step  308 , the write-in timings of the respective LED heads  14 Y–K are calculated from the calculated color-to-color misregistration quantities and the data shown in  FIG. 9 . At step  310 , the write-in timings of the respective LED heads  14 Y–K are set based on the result of calculation of the write-in timings. At step  312 , printing is started. 
   Toner images transferred to the continuous paper P are held between the photoreceptor drum  12  and the continuous paper P in the transfer process. The electrostatic force of attraction of the continuous paper P to the photoreceptor drum  12  is decreased because of the toner images existing therebetween. Further, the electrostatic force of attraction of the continuous paper P to the photoreceptor drum  12  is also varied depending on the coverages of the toner images already transferred to the continuous paper P at the upstream side. 
   Thus, the control unit  101  (see  FIG. 1 ) also performs control similar to the above-mentioned procedures (1) to (4) based on the coverages of toner images already transferred to the continuous paper P at the upstream side, thereby accurately preventing and correcting a color-to-color misregistration state. 
   For example, when transferring a cyan toner image on the photoreceptor drum  12 C onto the continuous paper P, the control unit  101  also performs controls similar to the above-mentioned procedures (1) to (4) based on the coverages of a yellow toner image and a magenta toner image which have already been transferred to the continuous paper P, thereby preventing and correcting a color-to-color misregistration state. Needless to say, the present invention is by no means limited to the above-described embodiment. 
   In the above embodiment, description has been made of a developing method in which after the photoreceptor drum  12  has been charged, a toner is adhered to that portion of the drum surface where the surface potential is decreased due to exposure, and then developed, that is, a so-called reversal development process in which the potential difference between a dark portion and a bright portion is reversed by applying a potential substantially equal to the surface potential to a developing electrode, and a toner charged with a polarity identical to that of the surface potential is used. However, it is also possible that a so-called positive development process may be used wherein a toner charged with a polarity opposite to that of a latent image is electrostatically adhered to a portion where surface potential has not been decreased due to exposure. 
   In the reversal development process, as the coverage of a toner image formed on the photoreceptor drum  12  is increased, the electrostatic force of attraction is decreased, and the transporting force with which the continuous paper P is held and transported by the photoreceptor drum  12  and the transfer roller  24  is decreased. Therefore, a correction is made for decreases in the electrostatic force of attraction and the transporting force when the coverage is increased. 
   In contrast thereto, in the positive development process, as the coverage of a toner image formed on the photoreceptor drum  12  is decreased, the electrostatic force of attraction is decreased, and the transporting force with which the continuous paper P is held and transported by the photoreceptor drum  12  and the transfer roller  24  is decreased. Therefore, a correction is made for decreases in the electrostatic force of attraction and the transporting force when the coverage is decreased, as opposed to the case of the reversal development process. 
   While in the above embodiment, the four-color (Y, M, C, K) laser printer  10  has been explained by way of example, it is to be understood that the present invention is by no means limited to a four-color laser printer, and is equally applicable to a five- or more-color laser printer in which other color or colors such as light magenta and the like have been added. Alternatively, the present invention is also applicable to a three- or less-color laser printer.