Patent Publication Number: US-9851660-B2

Title: Image forming apparatus with controlled separation voltage and transfer voltage

Description:
This application is a divisional of U.S. patent application Ser. No. 13/050,037, filed on Mar. 17, 2011, which claims the priority of Japanese Patent Application No. 2010-062656 filed on Mar. 18, 2010, the contents of which are herein incorporated by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus, and particularly relates to an image forming apparatus that forms an image by means of toner. 
     2. Description of Related Art 
     Hereinafter, the structure of a conventional general image forming apparatus is described.  FIG. 9  is a sectional constitutional view of the vicinity of a transfer roller  506  in a conventional image forming apparatus  500 . 
     The image forming apparatus  500  includes an intermediate transfer belt  502 , a driving roller  504 , a transfer roller  506 , and a separation member  508 . The intermediate transfer belt  502  is wound around the driving roller  504 . The driving roller  504  is rotated by a motor (not shown), whereby the intermediate transfer belt  502  is driven. The transfer roller  506  is provided so as to be opposed to the intermediate transfer belt  502 . 
     In the image forming apparatus  500 , a toner image formed on a photoreceptor (not shown) is transferred to the intermediate transfer belt  502  (primary transfer). Subsequently, the toner image transferred to the intermediate transfer belt  502  is transferred to paper passing between the intermediate transfer belt  502  and the transfer roller  506  (secondary transfer). The toner image is negatively charged. Further, the driving roller  504  is held at a ground potential, and the transfer roller  506  is held at a positive potential. The intermediate transfer belt  502  is held at a positive potential close to the ground potential. Under these conditions, the secondary transfer of the toner image is possible by an electric field generated between the intermediate transfer belt  502  and the transfer roller  506 . 
     Incidentally, in the image forming apparatus  500 , paper comes into contact with the transfer roller  506  held at the positive potential and therefore is positively charged. For this reason, the paper sticks to the intermediate transfer belt  502  through the electric field generated between the transfer roller  506  and the intermediate transfer belt  502 . Thereat, in the image forming apparatus  500 , the separation member  508  held at a negative potential is provided. Thereby, the paper is discharged by the separation member  508  and separated from the intermediate transfer belt  502 . 
     However, there has been a problem with the image forming apparatus  500  in that the quality of the toner image deteriorates when the potential of the separation member  508  is lowered for reliable separation of the paper from the intermediate transfer belt  502 . More specifically, when the potential of the separation member  508  is lowered, the potential difference between the transfer roller  506  and the separation member  508  increases, and therefore, a current flows from the transfer roller  506  to the separation member  508  via the paper. This causes a decrease in transfer current flowing from the transfer roller  506  to the intermediate transfer belt  502 , thereby resulting inadequate transfer of the toner image. Thus, the quality of the toner image deteriorates. 
     It is to be noted that as a conventional image forming apparatus, there is known, for example, an image forming apparatus described in Japanese Patent Application Laid-Open No. 2003-167450. In this image forming apparatus, the time of applying a transfer bias voltage is adjusted so as to improve the paper separation property from a photosensitive drum. In the image forming apparatus described in Japanese Patent Application Laid-Open No. 2003-167450, any separation members like the separation member  508  are not used to separate paper from the photosensitive drum, and the above-described problem of deterioration in image quality does not occur. 
     SUMMARY OF THE INVENTION 
     An image forming apparatus according to an embodiment of the present invention comprises: an image carrier that carries a toner image; a transfer member that is opposed to the image carrier, a transfer voltage being applied to the transfer member so that the toner image is transferred from the image carrier to a print medium passing between the transfer member and the image carrier; a first voltage application device that applies the transfer voltage to the transfer member; a separation member to which a separation voltage is applied so that the print medium is separated from the image carrier; a second voltage application device that applies the separation voltage to the separation member; a sensing device that senses an image density of the toner image; and a control section that controls a magnitude of the separation voltage based upon the image density of the toner image. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       This and other objects and features of the present invention will be apparent from the following description with reference to the accompanying drawings, in which: 
         FIG. 1  is a view showing the overall structure of an image forming apparatus according to an embodiment of the present invention; 
         FIG. 2  is an enlarged view of a conveyance channel from a pair of timing rollers to a fixing unit; 
         FIG. 3  is an enlarged view of the vicinity of a secondary transfer roller; 
         FIG. 4  is a plan view of a separation member seen from a direction of an arrow γ in  FIG. 3 ; 
         FIG. 5  is a flowchart showing an operation performed by a control section for transfer of a toner image to paper; 
         FIG. 6  is a flowchart showing an operation performed by the control section of the image forming apparatus according to a first modification for transfer of a toner image to paper; 
         FIG. 7  is a plan view of paper; 
         FIG. 8  is a flowchart showing an operation performed by the control section of the image forming apparatus according to a second modification for transfer of a toner image to paper; and 
         FIG. 9  is a sectional view of the vicinity of a transfer roller in a conventional image forming apparatus. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Configuration of Image Forming Apparatus 
     Hereinafter, an image forming apparatus according to an embodiment of the present invention is described with reference to the drawings.  FIG. 1  shows the overall structure of an image forming apparatus  1  according to the embodiment of the present invention.  FIG. 2  is an enlarged view of a conveyance channel R from a pair of timing rollers  19  to a fixing unit  20 .  FIG. 3  is an enlarged view of the vicinity of a secondary transfer roller  14 . 
     An image forming apparatus  1  is an electrophotographic color printer of a tandem type, which is configured so as to synthesize an image of four colors of Y (yellow), M (magenta), C (cyan) and K (black). The image forming apparatus  1  has a function of forming an image on paper (print medium) based upon image data read by a scanner, and as shown in  FIGS. 1 to 3 , the image forming apparatus  1  includes a printing section  2 , a paper feeding section  15 , a timing roller couple  19 , a fixing unit  20 , a printed-paper tray  21 , a control section  30 , a voltage application sections  31 ,  32 , a sensor (sensing device)  34 , a memory  35 , a touch panel  36 , a separation section  50  (cf.  FIGS. 2 and 3 ), and a separation claw  60  (cf.  FIG. 2 ). 
     The paper feeding section  15  serves to feed paper P piece by piece, and includes a paper tray  16  and a paper feeding roller  17 . A plurality of pieces of paper P to be subjected to printing are stacked and placed in the paper tray  16 . The paper feeding roller  17  takes out the paper from the paper tray  16  piece by piece. The pair of timing rollers  19  conveys the paper P, while adjusting the timing so that a toner image can be transferred to the paper P in the printing section  2 . It is to be noted that as shown in  FIG. 1 , the paper P is conveyed along the conveyance channel R in a direction of an arrow β. As shown in  FIGS. 1 and 2 , the conveyance channel R is made up of a plurality of guides. 
     The printing section  2  forms a toner image on the paper P fed from the paper feeding section  15 . The printing section  2  includes: an image forming section  22  ( 22 Y,  22 M,  22 C,  22 K); a transfer section  8  ( 8 Y,  8 M,  8 C,  8 K); an intermediate transfer belt (image carrier)  11 ; a driving roller  12 ; a driven roller  13 ; a secondary transfer roller (transfer member)  14 ; and a cleaning unit  18 . Further, the image forming section  22  ( 22 Y,  22 M,  22 C,  22 K) includes: a photosensitive drum  4  ( 4 Y,  4 M,  4 C,  4 K); a charger  5  ( 5 Y,  5 M,  5 C,  5 K); an exposure unit  6  ( 6 Y,  6 M,  6 C,  6 K); a development unit  7  ( 7 Y,  7 M,  7 C,  7 K); a cleaner  9  ( 9 Y,  9 M,  9 C,  9 K), and an eraser  10  ( 10 Y,  10 M,  10 C,  10 K). 
     The charger  5  charges the peripheral surface of the photosensitive drum  4 . The exposure unit  6  applies laser by control of the control section  30 . Thereby, an electrostatic latent image is formed on the peripheral surface of the photosensitive drum  4 . That is, the charger  5  and the exposure unit  6  serve as an electrostatic latent image forming device for forming an electrostatic latent image on the peripheral surface of the photosensitive drum  4 . 
     As shown in  FIG. 1 , the development unit  7  ( 7 Y,  7 M,  7 C,  7 K) includes a development roller  72  ( 72 Y,  72 M,  72 C,  72 K), a feeding roller  74  ( 74 Y,  74 M,  74 C,  74 K), a stirring roller  76  ( 76 Y,  76 M,  76 C,  76 K), and a housing section  78  ( 78 Y,  78 M,  78 C,  78 K). In  FIG. 1 , for the sake of simplicity of the drawing, only the development roller  72 Y, the feeding roller  74 Y, the stirring roller  76 Y, and the housing section  78 Y of the development unit  7 Y are provided with reference numerals. The housing section  78  constitutes a body of the development unit  7 , and houses the development roller  72 , the feeding roller  74  and the stirring roller  76 . Also, toner is stored in the housing section  78 . The stirring roller  76  stirs the toner inside the housing section  78  to negatively charge the toner. The feeding roller  74  feeds the negatively charged toner to the development roller  72 . The development roller  72  imparts the toner to the photosensitive drum  4 . Specifically, a negative development bias voltage is applied to the development roller  72  so as to form a development field between the photosensitive drum  4  and the development roller  72 . Since the toner is negatively charged, the toner moves from the development roller  72  to the photosensitive drum  4  under the influence of the development field. Further, the toner adheres to the photosensitive drum  4  based upon the electrostatic latent image formed on the photosensitive drum  4 . In this way, the electrostatic latent image is developed into a toner image on the photosensitive drum  4 . 
     The intermediate transfer belt  11  is extended between the driving roller  12  and the driven roller  13 , and the toner image developed on the photosensitive drum  4  is transferred to the intermediate transfer belt  11  (primary transfer). The transfer section  8  is arranged so as to be opposed to the inner peripheral surface of the intermediate transfer belt  11 . When a primary transfer voltage is applied to the transfer section  8 , the toner image formed on the photosensitive drum  4  is transferred to the intermediate transfer belt  11 . The cleaner  9  collects toner that remains on the peripheral surface of the photosensitive drum  4  after the primary transfer. The eraser  10  removes charge from the peripheral surface of the photosensitive drum  4 . The driving roller  12  is rotated by an intermediate transfer belt driving section (not shown in  FIG. 1 ) to drive the intermediate transfer belt  11  in a direction of an arrow α. In this manner, the intermediate transfer belt  11  conveys the toner image to the secondary transfer roller  14 . Therefore, the intermediate transfer belt  11  functions as an image carrier for carrying and delivering a toner image. The sensor  34  is provided so as to be opposed to the intermediate transfer belt  11  on the upstream side from the secondary transfer roller  14  in a direction of an arrow α, and senses the density of the toner image. 
     The secondary transfer roller  14  is opposed to the intermediate transfer belt  11 . When a transfer voltage is applied to the secondary transfer roller  14 , the toner image is transferred from the intermediate transfer belt  11  to the paper P passing between the intermediate transfer belt  11  and the secondary transfer roller  14  (secondary transfer). More specifically, the driving roller  12  is held at a ground potential. Further, the intermediate transfer belt  11  is in contact with the driving roller  12 , and thereby held at a positive potential close to the ground potential. The voltage application section  31  applies a positive transfer voltage to the secondary transfer roller  14  such that the potential of the secondary transfer roller  14  becomes higher than those of the driving roller  12  and the intermediate transfer belt  11 . Since the toner image is negatively charged, the toner image is transferred from the intermediate transfer belt  11  to the paper P through the electric field generated between the driving roller  12  and the secondary transfer roller  14 . 
     After the secondary transfer of the toner image to the paper P, the cleaning unit  18  removes toner that remains on the intermediate transfer belt  11 . 
     The paper P with the toner image transferred thereto is conveyed to the fixing unit  20 . The fixing unit  20  performs a heating treatment and a pressure treatment on the paper P to fix the toner image to the paper P. The printed paper P is placed in the printed-paper tray  21 . 
     The separation section  50  is a separation member that separates the paper P from the intermediate transfer belt  11 , and is provided in the conveyance channel R, at a position downstream from the intermediate transfer belt  11  and the secondary transfer roller  14  in the conveying direction, and on the opposite side of the conveyance channel R to the intermediate transfer belt  11 , as shown in  FIGS. 2 and 3 . As shown in  FIG. 3 , the separation section  50  is made up of a separation member  51 , a base  52 , a spacer  54  and a protective member  56 . 
     The base  52  is bent into L shape in cross section, and is provided above the secondary transfer roller  14 . The base  52  constitutes part of a body of the image forming apparatus  1 , and is made of an insulating material. The separation member  51  is a metal plate of stainless or the like mounted on the upper surface of the base  52 , and is bent into L shape in cross section, following the shape of the base  52 .  FIG. 4  is a plan view of the separation member  51  seen from a direction of an arrow γ in  FIG. 3 . As shown in  FIG. 4 , the separation member  51  has a sawtooth edge in its tip portion opposed to the conveyance channel R. 
     The spacer  54  is an insulating member provided between the base  52  and the separation member  51 , and prevents the tip of the separation member  51  from coming into contact with the base  52 . Thereby, the tip of the separation member  51  is prevented from being damaged. The protective member  56  is an insulating member provided on the top surface of the separation member  51 , and protects the tip of the separation member  51 . 
     A voltage application section  32  applies a voltage to the separation member  51  such that the potential of the intermediate transfer belt  11  becomes a value between the potential of the separation member  51  and the potential of the secondary transfer roller  14 . Then, the paper P is separated from the intermediate transfer belt  11  by electrical force. More specifically, the driving roller  12  is held at the ground potential. Further, the intermediate transfer belt  11  is in contact with the driving roller  12 , and thereby held at a positive potential close to the ground potential. The voltage application section  32  applies a negative separation voltage to the separation member  51  such that the potential of the separation member  51  becomes smaller than the potentials of the driving roller  12  and the intermediate transfer belt  11 . The paper P is positively charged by contact with the secondary transfer roller  14 . This causes discharge from the sawtooth edge of the separation member  51 , thereby removing the charge of the paper P. Consequently, the paper P is separated from the intermediate transfer belt  11 . 
     As shown in  FIG. 2 , the separation claw  60  is provided so as to be opposed to the intermediate transfer belt  11 , at a position downstream from the portion where the intermediate transfer belt  11  and the secondary transfer roller  14  are opposed to each other in the driving direction of the intermediate transfer belt  11 . If the paper P is wound around and conveyed by the intermediate transfer belt  11  without being separated by the separation section  50 , the separation claw  60  separates the paper P from the intermediate transfer belt  11  by physical force. 
     The touch panel  36  is an input unit for receiving an input when a user touches the screen. The touch panel  36  also serves as an information receiving device for receiving information on the type (thin paper, ordinary paper or thick paper) of the paper P. Specifically, in the image forming apparatus  1 , the touch panel  36  receives information on the thickness of the paper P as the information on the type of the paper P. 
     The control section  30  controls an overall operation of the image forming apparatus  1 , and is realized by a CPU. Especially, in the image forming apparatus  1  according to the present embodiment, the control section  30  controls the magnitude of the separation voltage to be applied by the voltage application section  32  to the separation member  51 , based upon the image density sensed by the sensor  34 . Further, the control section  30  controls the magnitude of the transfer voltage to be applied by the voltage application section  31  to the secondary transfer roller  14 , based upon the information on the type of the paper P received by the touch panel  36 . Specifically, the memory  35  stores a table as shown by Table 1. 
     
       
         
           
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                 Image Density 
               
            
           
           
               
               
               
               
               
               
               
               
               
               
            
               
                   
                 not less than 
                 not less than 
                 not less than 
                 not less than 
                 not less than 
                 not less than 
                 not less than 
                 not less than 
                   
               
               
                 Separation 
                 0% and less 
                 10% and less 
                 20% and less 
                 30% and less 
                 40% and less 
                 50% and less 
                 60% and less 
                 70% and less 
               
               
                 Voltage (−V) 
                 than 10% 
                 than 20% 
                 than 30% 
                 than 40% 
                 than 50% 
                 than 60% 
                 than 70% 
                 than 80% 
                 less than 80% 
               
               
                   
               
               
                 Thin Paper 
                 4000 
                 3000 
                 3000 
                 2500 
                 2500 
                 2000 
                 2000 
                 1500 
                 1000 
               
               
                 Ordinary 
                 3000 
                 2500 
                 2500 
                 2500 
                 2000 
                 2000 
                 1500 
                 1000 
                 1000 
               
               
                 Paper 
               
               
                 Thick Paper 
                 2500 
                 2000 
                 2000 
                 1500 
                 1000 
                 1000 
                 1000 
                 1000 
                 1000 
               
               
                   
               
            
           
         
       
     
     Table 1 shows the relation among the image density, the type of the paper P and the separation voltage. As shown in Table 1, as the image density increases, the separation voltage shall be set smaller so that the difference between the potential of the intermediate transfer belt  11 , which is close to the ground potential, and the potential of the separation member  51  will be smaller. Further, as the thickness of the paper P increases, the separation voltage shall be set smaller so that the difference between the potential of the intermediate transfer belt  11 , which is close to the ground potential, and the potential of the separation member  51  will be smaller. The control of the magnitude of the separation voltage will be described in the following paragraphs. Hereinafter, that the separation voltage is “large” or “small” means that the absolute value (magnitude) of the separation voltage is large or small. 
     Tests 
     The present inventors performed two tests described below in order to decide a method for controlling the magnitude of the separation voltage. In the first test, using the image forming apparatus  1  shown in  FIG. 1 , a toner image with a relatively high image density (hereinafter referred to as a high-density image) and a toner image with a relatively low image density (hereinafter referred to as a low-density image) were formed on three types of paper P, which are thin paper, ordinary paper and thick paper, with different separation voltages (0 V, 1000 V, 2000 V and 3000 V) applied. Then, in each case, whether or not the paper P has been separated from the intermediate transfer belt  11  (hereinafter referred to as separation performance) was checked. Table 2 shows test results. In Table 2, ◯ indicates that the paper P was separated without any difficulty, Δ indicates that the paper P was separated with such a little difficulty not to cause any practical problems, and × indicates that the paper P could not be separated. 
     
       
         
           
               
               
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                   
                 Low-Density Image 
                 High-Density Image 
               
               
                 Separation 
                 Separation Voltage (−V) 
                 Separation Voltage (−V) 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Performance 
                 0 
                 1000 
                 2000 
                 3000 
                 0 
                 1000 
                 2000 
                 3000 
               
               
                   
               
               
                 Thin Paper 
                 x 
                 x 
                 x 
                 ∘ 
                 x 
                 x 
                 Δ 
                 ∘ 
               
               
                 Ordinary Paper 
                 x 
                 x 
                 ∘ 
                 ∘ 
                 x 
                 ∘ 
                 ∘ 
                 ∘ 
               
               
                 Thick Paper 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
               
               
                   
               
            
           
         
       
     
     As shown in Table 2, it was found that a larger separation voltage is required when the thickness of the paper P becomes smaller. It is, therefore, desired that the separation voltage is controlled to become larger with decreases in thickness of the paper P, as shown in Table 1. 
     Moreover, in forming the high-density image, satisfactory separation performance of the paper P was ensured even with a smaller separation voltage than in forming the low-density image. The reason is as follows. The paper P with the high-density image formed thereon sticks to the intermediate transfer belt  11  with weaker force than the paper P with the low-density image formed thereon because a larger amount of toner is present between the paper P with the high-density image formed thereon and the intermediate transfer belt  11  than between the paper P with the lower-density image formed thereon and the intermediate transfer belt  11 . It is, therefore, desired that the separation voltage is controlled to become smaller with increases in image density, as shown in Table 1. 
     In the second test, using the image forming apparatus  1  shown in  FIG. 1 , a toner image with a relatively high image density (hereinafter referred to as a high-density image) and a toner image with a relatively low image density (hereinafter referred to as a low-density image) were formed on three types of paper P, which are thin paper, ordinary paper and thick paper, with different separation voltages (0 V, 1000 V, 2000 V and 3000 V) applied. Then, in each case, whether or not the toner image has been transferred to the paper P satisfactorily (hereinafter referred to as transfer performance) was checked. Table 3 is shows test results. In Table 3, ◯ indicates that the toner image was transferred satisfactorily, and × indicates that satisfactory toner image transfer could not be done. 
     
       
         
           
               
               
               
             
               
                 TABLE 3 
               
             
            
               
                   
               
               
                   
                 Low-Density Image 
                 High-Density Image 
               
               
                 Transfer 
                 Separation Voltage (−V) 
                 Separation Voltage (−V) 
               
            
           
           
               
               
               
               
               
               
               
               
               
            
               
                 Performance 
                 0 
                 1000 
                 2000 
                 3000 
                 0 
                 1000 
                 2000 
                 3000 
               
               
                   
               
               
                 Thin Paper 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 x 
               
               
                 Ordinary Paper 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 x 
               
               
                 Thick Paper 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 ∘ 
                 x 
                 x 
               
               
                   
               
            
           
         
       
     
     As shown in Table 3, in forming the low-density image, satisfactory transfer performance could be achieved under every condition. On the other hand, in forming the high-density image, satisfactory transfer performance could not be obtained when the separation voltage was large. The fails in transfer are due to a current flow from the secondary transfer roller  14  to the separation member  51  that occurs when the separation voltage is large. It was found from the second test that a large separation voltage cannot be applied in the cases of forming the high-density image. Accordingly, also from the viewpoint of the transfer performance, it is desired that the separation voltage is controlled to become smaller with increases in image density of the toner image, as shown in Table 1. 
     Operation of Image Forming Apparatus 
     Hereinafter, the operation of the image forming apparatus  1  configured as above is described with reference to the drawings.  FIG. 5  is a flowchart showing an operation performed by the control section  30  for transfer of a toner image to the paper P. 
     The present process is started when the user issues a printing instruction with the touch panel  36 . The control section  30  obtains information on the type of the paper P, which is inputted on the touch panel  36 , to identify the thickness of the paper P (step S 1 ). Next, the control section  30  makes the printing section  2  form a toner image. 
     Next, the control section  30  makes the sensor  34  sense the image density of the toner image transferred to the intermediate transfer belt  11  (step S 2 ). The control section  30  then decides a separation voltage referring to Table 1, based upon the thickness of the paper P identified at step S 1  and the image density sensed at step S 2  (step S 3 ). 
     Next, the control section  30  makes the paper feeding section  15  and the pair of timing rollers  19  convey the paper P to the secondary transfer roller  14  for secondary transfer of the toner image to the paper P, while making the voltage application section  32  apply the separation voltage to the separation member  51  (step S 4 ). Thereby, even if the paper P is positively charged, the charge of the paper P is removed in the separation member  51  while the paper P is passing between the secondary transfer roller  14  and the intermediate transfer belt  11 . As a result, the paper P is separated from the intermediate transfer belt  11  and conveyed to the fixing unit  20 . Subsequently, the fixing unit  20  performs a heating treatment and a pressure treatment on the paper P to fix the toner image to the paper P. The paper P is then ejected onto the printed-paper tray  21 . 
     Effect 
     In the image forming apparatus  1  configured as above, it is possible to reliably separate the paper P from the intermediate transfer belt  11  without causing deterioration in image quality. More specifically, as shown in Table 3, when the image density of the toner image becomes higher, deterioration in transfer performance is apt to occur due to a current flow from the secondary transfer roller  14  to the separation member  51 , and accordingly, deterioration in image quality is apt to occur. On the other hand, as shown in Table 2, when the image density of the toner image becomes higher, satisfactory separation performance can be achieved even with a small separation voltage. Therefore, in the image forming apparatus  1 , the separation voltage is controlled to become smaller with increases in image density of the toner image, as shown in Table 1. Accordingly, when the image density of the toner image is low, which means that the paper P is difficult to separate from the intermediate transfer belt, a relatively high separation voltage is used to separate the paper P. Further, when the image density of the toner image is high, a current is apt to flow from the secondary transfer roller  14  to the separation member  51 , and it is thereby possible to separate the paper P by use of a relatively low separation voltage. Thus, according to the image forming apparatus  1 , it is possible to reliably separate the paper P from the intermediate transfer belt  11  without causing deterioration in image quality. 
     Further, as is apparent from Table 2, a larger separation voltage is required as the thickness of the paper P becomes smaller. This is because a thinner paper P is more apt to be wound around the intermediate transfer belt  11 . Therefore, in the image forming apparatus  1 , the separation voltage is controlled to become larger with decreases in thickness of the paper P, as shown in Table 1. This allows reliable separation of the paper P from the intermediate transfer belt  11 . 
     First Modification 
     Hereinafter, an image forming apparatus  1   a  according to a first modification is described. In the image forming apparatus  1 , the control section  30  makes the voltage application section  31  apply a transfer voltage with a constant magnitude to the secondary transfer roller  14 . On the other hand, in the image forming apparatus  1   a,  the control section  30  changes the magnitude of the transfer voltage based upon the magnitude of the separation voltage. Specifically, the memory  35  stores a table shown in Table 4. 
     
       
         
           
               
               
               
             
               
                   
                 TABLE 4 
               
             
            
               
                   
                   
               
               
                   
                 Transfer 
                 Separation Voltage (−V) 
               
            
           
           
               
               
               
               
               
               
               
            
               
                   
                 Voltage (V) 
                 1000 
                 1500 
                 2000 
                 2500 
                 3000 
               
               
                   
                   
               
               
                   
                 Thin Paper 
                 1800 
                 1850 
                 1950 
                 2100 
                 2300 
               
               
                   
                 Ordinary Paper 
                 1900 
                 2000 
                 2100 
                 2300 
                 2500 
               
               
                   
                 Thick Paper 
                 2100 
                 2200 
                 2300 
                 2500 
                 2800 
               
               
                   
                   
               
            
           
         
       
     
     Table 4 shows the relation among the type of the paper P, the separation voltage and the transfer voltage to achieve satisfactory transfer. As shown in Table 4, the transfer voltage shall be larger with increases in separation voltage. In other words, the larger the difference between the potential of the intermediate transfer belt  11 , which is close to the ground potential, and the potential of the separation member  51  is, the larger the potential difference between the intermediate transfer belt  11  and the secondary transfer roller  14  shall be. Further, the transfer voltage shall be larger with increases in thickness of the paper P. In other words, the larger the thickness of the paper P is, the larger the potential difference between the intermediate transfer belt  11  and the separation member  51  shall be. According to the first modification, therefore, the control section  30  further controls the transfer voltage based upon the thickness of the paper P and the separation voltage. Hereinafter, the operation of the image forming apparatus  1   a  is described with reference to the drawings.  FIG. 6  is a flowchart showing an operation performed by the control section  30  of the image forming apparatus  1   a  for transfer of a toner image to the paper P. 
     Since steps S 1  to S 3  in  FIG. 6  are the same as steps S 1  to S 3  in  FIG. 5 , the descriptions thereof are not given. At step S 13 , the control section  30  decides a transfer voltage referring to Table 4, based upon the thickness of the paper P identified at step S 1  and the separation voltage decided at step S 3  (step S 13 ). Subsequently, the process goes to step S 14 . 
     Next, the control section  30  makes the voltage application section  31  apply the transfer voltage to the secondary transfer roller  14  and makes the paper feeding section  15  and the pair of timing rollers  19  convey the paper P to the secondary transfer roller  14  so that a toner image is transferred to the paper P, while making the voltage application section  32  apply the separation voltage to the separation member  51  (step S 14 ). Subsequently, the fixing unit  20  performs a heating treatment and a pressure treatment on the paper P to fix the toner image to the paper P. The paper P is then ejected onto the printed-paper tray  21 . 
     The image forming apparatus  1   a  has improved transfer performance. More specifically, as shown in Table 3, when the separation voltage becomes higher, the transfer voltage becomes insufficient, which may result in a decrease in transfer performance. In the image forming apparatus  1   a,  therefore, the transfer voltage is controlled to become larger with increases in separation voltage. This inhibits a decrease in transfer performance due to insufficient transfer voltage when the separation voltage is large. 
     Second Modification 
     Hereinafter, an image forming apparatus  1   b  according to a second modification is described. In the image forming apparatus  1 , the control section  30  holds the separation voltage constant while one piece of paper P is passing between the intermediate transfer belt  11  and the secondary transfer roller  14 . On the other hand, in the image forming apparatus  1   b,  the control section  30  changes the separation voltage while one piece of paper P is passing between the intermediate transfer belt  11  and the secondary transfer roller  14 .  FIG. 7  is a plan view of the paper P. 
     A toner image is formed on the paper P shown in  FIG. 7 . An area where the toner image is formed is defined as an area A 1 , and an area in the leading portion in a conveying direction ahead of the area A 1  is defined as an area A 2 . The toner image is not formed in the area A 2 . Further, a boundary between the area A 1  and the area A 2  is defined as a boundary B. With respect to the area A 1  where a toner image is formed, it is possible to separate the paper P from the intermediate transfer belt  11  even with a relatively low separation voltage. On the other hand, in the area A 2  where no toner image is formed, a relatively high separation voltage is required to separate the paper P from the intermediate transfer belt  11 . In the image forming apparatus  1   b,  therefore, the control section  30  controls the separation voltage to have different magnitudes between while the area A 2  is passing by the separation member  51  and while the area A 1  is passing by the separation member  51 . Assuming the magnitude of the separation voltage applied to the separation member  51  while the area A 2  is passing by the separation member  51  as a first magnitude and assuming the magnitude of the separation voltage applied to the separation member  51  while the are A 1  is passing by the separation member  51  as a second magnitude, the control section  30  controls the separation voltage such that the second magnitude is smaller then the first magnitude. In other words, the potential difference between the intermediate transfer belt  11  and the separation member  51  while the area A 1  is passing by the separation member  51  is made smaller than the potential difference between the intermediate transfer belt  11  and the separation member  51  while the area A 2  is passing by the separation member  51 . Hereinafter, the operation of the image forming apparatus  1   b  is described with reference to the drawings.  FIG. 8  is a flowchart showing an operation performed by the control section  30  of the image forming apparatus  1   b  for transfer of a toner image to the paper P. 
     Since step S 1  in  FIG. 8  is the same as step S 1  in  FIG. 5 , the description thereof is not given. At step S 21 , the control section  30  identifies the leading edge of the toner image in the conveying direction when the control section  30  senses that the image density has increased from 0% for the first time since the start of sensing (step S 21 ). Next, the control section  30  calculates a distance D of the boundary B between the area A 1  and the area A 2  from the leading edge of the paper P, based upon the time of sensing the leading edge of the toner image (step S 22 ). Subsequently, the process goes to step S 2 . Since step S 2  in  FIG. 8  is the same as step S 2  in  FIG. 5 , the description thereof is not given. 
     At step S 23 , the control section  30  decides the first magnitude of the separation voltage referring to Table 1, based upon the thickness of the paper P identified at step S 1  (step S 23 ). It is to be noted that the image density that is a factor of the decision of the first magnitude of the separation voltage is not less than 0% and less than 10%. 
     Next, the control section  30  decides the second magnitude of the separation voltage referring to Table 1, based upon the thickness of the paper P identified at step S 1  and the image density sensed at step  2  (step S 24 ). 
     Next, the control section  30  makes the paper feeding section  15  and the pair of timing rollers  19  convey the paper P to the secondary transfer roller  14  so as to start secondary transfer of the toner image to the paper P, while making the voltage application section  32  apply the separation voltage with the first magnitude to the separation member  51  (step S 25 ). 
     Next, the control section  30  determines whether or not the boundary B has reached the separation member  51  (step S 26 ). The determination in step S 26  is performed, for example, based upon the distance D calculated at step S 22  and a distance that the pair of timing rollers  19  conveyed the paper P. When the boundary B has not reached the separation member  51 , the process returns to step S 26 . On the other hand, when the boundary B has reached the separation member  51 , the process goes to step S 27 . 
     When the boundary B has reached the separation member  51 , the control section  30  makes the voltage application section  32  apply the separation voltage with the second magnitude to the separation member  51  (step S 27 ). Subsequently, the fixing unit  20  performs a heating treatment and a pressure treatment on the paper P to fix the toner image to the paper P. The paper P is then ejected onto the printed-paper tray  21 . 
     In the image forming apparatus  1   b,  it is possible to separate the paper P from the intermediate transfer belt  11  more reliably without causing deterioration in image quality. More specifically, in the area A 1  where the toner image is formed, the paper P can be separated from the intermediate transfer belt  11  with a smaller separation voltage as compared with the area A 2  where the toner image is not formed. On the other hand, since the toner image is formed in the area A 2 , a current is apt to flow from the secondary transfer roller  14  to the separation member  51  as compared with the area A 1 . Therefore, in the image forming apparatus  1   b,  the separation voltage applied while the area A 1  is passing by the separation member  51  has a smaller magnitude than that while the area A 2  is passing by the separation member  51 . Thereby, high separation performance can be achieved in the area A 2  while high transfer performance can be achieved in the area A 1 . 
     It should be noted that in the image forming apparatus  1 ,  1   a,    1   b,  the transfer section  8  may be a roller. 
     The image forming apparatuses  1 ,  1   a  and  1   b  may be of a type wherein the toner image is directly transferred from the photosensitive drum  4  to the paper P not via the intermediate transfer belt  11 . In this case, the photosensitive drum  4  functions as the image carrier. 
     Further, although the separation voltage is decided referring to Table 1 in the image forming apparatuses  1 ,  1   a  and  1   b,  the separation voltage may be, for example, calculated by use of equation (1) shown below.
 
 V= 3000− a×b    (1)
 
     V: separation voltage (−V) 
     a: constant 
     b: image density (%) 
     In the image forming apparatuses  1 ,  1   a  and  1   b,  the separation voltage is decided also based upon the thickness of the paper P. However, in the image forming apparatuses  1 ,  1   a  and  1   b,  the separation voltage may be decided based upon the temperature, the humidity or the resistance value of the paper P, in addition to the thickness of the paper P. 
     The separation member  51  may have an edge with needles, a brush edge or the like, other than the sawtooth edge shown in  FIG. 4 . Further, the separation member  51  may be made of a conductive cloth. 
     In the image forming apparatuses according to the embodiments, it is possible to reliably separate a print medium from an image carrier without causing deterioration in image quality. 
     Although the present invention has been described in connection with the preferred embodiments, it is to be noted that various changes and modifications are possible to those who are skilled in the art. Such changes and modifications are to be understood as being within the scope of the present invention.