Abstract:
An image forming apparatus includes an image bearing member which bears a toner image, a transferring member which forms the image bearing member and a nip and transfers the toner image onto a transferring material, a bias applying unit for applying a transferring bias to the transferring member, the bias applying unit changing from the transferring bias to a low bias lower than the transferring bias at the rear end portion of the transferring material, and a control unit for controlling a position at which the transferring bias is changed to the low bias.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus such as a copying machine or a printer which uses an electrophotographic system or an electrostatic recording system. 
     2. Related Background Art 
     FIG. 11 shows an image forming apparatus of the electrophotographic system. 
     In the figure, the image forming apparatus includes a drum-shaped electrophotographic photosensitive member (hereinafter referred to as “photosensitive drum”)  1  which rotates in a direction indicated by an arrow a (clockwise). A charging roller  2 , an exposing apparatus  3 , a developing apparatus  4 , a transfer roller  5  and a cleaning apparatus  6  are disposed around the photosensitive drum  1 . 
     Upstream of a transferring nip portion N between the photosensitive drum  1  and the transfer roller  5  in a conveying direction of a transferring material P, a sheet feeding cassette  7 , a sheet feeding roller  8 , a pre-feed sensor  9 , a pair of resist rollers  10 , a top sensor  11  and a transfer guide  12  are disposed. On the other hand, downstream of the transferring nip portion N in the conveying direction of the transferring material P, an electricity erasing needle  13 , a conveying guide  14 , a fixing apparatus  15  and a pair of sheet discharging rollers  16  are disposed. 
     Also, the image forming apparatus can perform duplex image formation (duplex printing), and a pair of reverse rollers  18 , a pair of sheet re-feeding rollers  19  and a sheet re-feeding sensor  20  for duplex image formation are disposed in a duplex conveying path  17 . 
     The photosensitive drum  1  is formed of a negatively charged OPC photosensitive member which is rotationally driven in the direction indicated by the arrow a at a given process speed in a conventional example. 
     The charging roller  2  is brought in contact with a surface of the photosensitive drum  1  by a predetermined pressing force and charges the photosensitive drum  1  to a predetermind polarity and potential by a charging bias applied from a charging bias power supply (not shown). 
     The exposing apparatus  3  includes a laser diode that emits a laser beam, a collimator lens, a polygon mirror, an fθ lens, etc., not shown, and scans the photosensitive drum  1 , which has been uniformly charged by the charging roller  2 , with an outgoing laser beam L, which has been on/off-controlled in accordance with inputted image information, in a direction perpendicular to the rotating direction of the photosensitive drum  1  and then exposes light. The exposure allows charges on a portion of the photosensitive drum  1  which has been scanned with the laser beam L to be removed, and an electrostatic latent image to be formed on the surface of the photosensitive drum  1 . 
     The developing apparatus  4  includes a rotatable developing sleeve  4   a  inside of which a magnet roller (not shown) is fixed. The developing apparatus  4  coats a developer (toner) on a developing sleeve  4   a  into a thin layer and attaches the toner onto the electrostatic latent image formed on the surface of the photosensitive drum  1  at a developing position to develop (visualize) the electrostatic latent image as a toner image. The developing sleeve  4   a  is applied with a developing bias from a developing bias power supply (not shown). 
     The transfer roller  5  is brought in contact with the surface of the photosensitive drum  1  by a predetermied pressing force to form the transferring nip portion N, and transfers the toner image on the surface of the photosensitive drum  1  onto the transferring material P at the transferring nip portion N defined between the photosensitive drum  1  and the transfer roller  5 . The transfer roller  5  is rotated in a direction indicated by an arrow b (counterclockwise). 
     The fixing apparatus  15  includes a fixing roller  15   a  and a pressure roller  15   b . The fixing apparatus  15  heats and pressurizes the transferring material P onto which the toner image has been transferred between the fixing roller  15   a  and the pressure roller  15   b  to thermally fix the toner image onto the surface of the transferring material P. 
     Subsequently, the image forming operation of the above-described image forming apparatus will be described. 
     Upon the issuance of an image forming operation start signal, the transferring materials P within the cassette  7  are fed one by one by the sheet feeding roller  8  and conveyed up to the pair of resist rollers  10 . During this operation, the conveyance of the transferring materials P is detected by the pre-feed sensor  9 . 
     On the other hand, the photosensitive drum  1  is rotationally driven in the direction indicated by the arrow a by driving means (not shown) and charged to the predetermined potential by the charging roller  2 . Then, the laser beam L corresponding to an image signal is irradiated onto the photosensitive drum  1  from the exposing apparatus  3 , and the potential on the photosensitive drum  1  at a portion onto which the laser beam L has been irradiated drops to form the electrostatic latent image. Then, the toner is attached onto the electrostatic latent image formed on the photosensitive drum  1  by the developing sleeve  4   a  of the developing apparatus  4  to which the developing bias having the same polarity as the charging polarity (negative polarity) of the photosensitive drum  1  is applied, to thereby visualize the electrostatic latent image as the toner image. 
     Then, after a leading end of the transferring material P is detected by the top sensor  11 , the transferring material P is fed by the pair of resist rollers  10  to the transferring nip portion N between the photosensitive drum  1  and the transfer roller  5  through the transfer guide  12  in synchronism with the rotation of the photosensitive drum  1 . Then, the transfer roller  5  is applied with a transferring bias having the reverse polarity (positive polarity) of the toner from the transferring bias power supply (not shown), and the toner image is transferred onto the transferring material P from the photosensitive drum  1 . 
     Then, the transferring material P onto which the toner image has been transferred is electricity-erased by the electricity erasing needle  13  to which charges having the reverse polarity (negative polarity) of the transfer roller  5  are given, and the transferring material P is separated from the photosensitive drum  1  by the own weight of the transferring material P. The transferring material P which has been separated from the photosensitive drum  1  is conveyed to the fixing apparatus  15  through the conveying guide  14 , and the transferred toner image is thermally fixed onto the surface of the transferring material P due to heating and pressurizing by the fixing roller  15   a  and the pressure roller  12   b  of the fixing apparatus  15 . Then, the transferring material P is discharged to the external from the pair of sheet discharging rollers  16 . 
     On the other hand, a residual toner (untransferred toner) and other attachments are removed from the surface of the photosensitive drum  1  from which the toner image has been transferred and cleaned by the cleaning apparatus  6 , and the photosensitive drum  1  is put to succeeding image formation. 
     Also, at the time of duplex image formation (double-sided print), the transferring material P which has been discharged from the fixing apparatus  15  after the image has been formed on a first surface (front surface) of the transferring material P is reversed by the reverse rollers  18  and conveyed to the duplex conveying path  17 . Then, the transferring material P is allowed to pass through the sheet re-feeding sensor  20  by the sheet re-feeding rollers  19  and again introduced to the pair of resist rollers  10 . Thereafter, another image is formed on a second surface (back surface) of the transferring material P in the same manner as the first surface. 
     Note that at the time of the above-described image formation, the transfer roller  5  is applied with a transferring bias Vtr required for transferring the toner image from the transferring bias power supply (not shown) to give charges to the transferring material P that passes through the transferring nip portion N between the photosensitive drum  1  and the transfer roller  5  so that the toner image is transferred onto the transferring material P from the photosensitive drum  1 . After the toner image has been transferred onto the transferring material P, a voltage applied to the transfer roller  5  is turned off or changed to a low bias Vlow lower than the transferring bias Vtr, to thereby prevent a drum memory or a sheet mark from occurring at the time of non-transferring. 
     Assuming that an image forming area (printable area) S of the transferring material P is, as shown in FIG. 12, b mm (for example, 5 mm) inside of a leading end P 1 , a rear end P 2 , and right and left ends P 3 , respectively, the changing of the bias from the transferring bias Vtr after transferring to the low bias Vlow is conducted, as shown in FIG. 13, in a non-image area S′ inside of the rear end P 2  of the transferring material P by a distance c (c&lt;b) mm. In this example, the conveying direction of the transferring material P is a direction indicated by an arrow A. 
     As described above, in the case of changing the transferring bias, a timing of changing from the transferring bias to the low bias at the rear end P 2  of the transferring material P is decided by reckoning a time at which the rear end P 2  of the transferring material P reaches the transferring nip portion N from a time at which the rear end P 2  of the transferring material P passes through the top sensor  11  upstream of the transferring nip portion N, a distance between the top sensor  11  and the transferring nip portion N, and a conveying speed of the transferring material P backward. 
     However, the distance between the top sensor  11  and the transferring nip portion N is slightly varied by the respective manufactured image forming apparatuses, and the conveying speed of the transferring material P also depends on the outer diameter of the transfer roller  5 , the sort of the transferring material P, the printing ratio, etc., and slightly varies. For that reason, there arises such a problem that a timing of changing from the transferring bias to the low transferring bias is slightly shifted. 
     Also, the top sensor  11  includes, as shown in FIG. 14, a sensor lever  21  movable by contact with the transferring material P that passes through the sensor lever  21 , and a photo-interrupter  22  that detects the moved sensor lever  21  in a non-contact manner, and detects the leading end and the rear end of the transferring material P that passes through the top sensor  11 . 
     However, in the top sensor  11 , since the movable range of the sensor lever  21  depends on which portion of the sensor lever  21  the transferring material P passes through, there arises such a problem that an error occurs in a period of time until the photo-interrupter  22  detects the sensor lever  21 , to thereby shift the timing of changing the transferring bias. 
     As described above, if the timing of changing the transferring bias is shifted, for example, in the case where the changing timing is shifted toward the rear end side of the transferring material P, an excessive transferring current flows in the photosensitive drum  1  side at an edge portion of the rear end of the transferring material P. For that reason, the potential fluctuates due to the excessive current that flows at the rear end of the transferring material P on the photosensitive drum  1 , as a result of which there occurs a phenomenon that the potential fluctuation appears as a memory or a stripe on the photosensitive drum  1  at the time of printing a next transferring material P (hereinafter referred to as “rear end memory”). 
     On the contrary, in the case where the timing of changing the transferring bias is shifted toward the inside from the rear end of the transferring material P, the timing of changing the transferring bias is applied to the image forming area, and for example, at the time of printing the second surface of the transferring material P (a rear-surface printing at the time of the duplex image formation) or printing a high-resistant transferring material (paper) P, the transferring current is short when the bias is changed to the low transferring bias, thereby leading to such a problem that the toner image scatters. 
     In particular, the timing of changing the transferring bias is more shifted as the process speed becomes higher, and the high-speed printing operation of the image forming apparatus in recent years makes it difficult to provide latitude to that shift in the non-image area at the rear end of the transferring material P. 
     SUMMARY OF THE INVENTION 
     The present invention has been made under the above circumstances, and therefore an object of the present invention is to provide an image forming apparatus which does not allow any memory corresponding to a rear end of a transferring material to occur. 
     Another object of the present invention is to provide an image forming apparatus which does not allow a toner image on the rear end of the transferring material to scatter. 
     Still another object of the present invention is to provide an image forming apparatus which is capable of changing a transferring bias at an optimum timing. 
     Yet still another object of the present invention is to provide an image forming apparatus, comprising: 
     an image bearing member which bears a toner image; 
     a transferring member which forms the image bearing member and a nip and transfers the toner image onto a transferring material; 
     bias applying means for applying a transferring bias to the transferring member, the bias applying means changing from the transferring bias to a low bias lower than the transferring bias at the rear end portion of the transferring material; and 
     control means for controlling a position at which the transferring bias is changed to the low bias. 
     Yet still other objects of the present invention will become apparent from the following description. 
     These and other objects, features and advantages of this invention will become more fully apparent from the following detailed description taken with the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a schematic structural diagram showing a transferring bias control system of an image forming apparatus in accordance with a first embodiment of the present invention; 
     FIG. 2 is a side view showing a transferring nip portion between a photosensitive drum and a transfer roller in the image forming apparatus in accordance with the first embodiment of the present invention; 
     FIG. 3 is a front view showing the transferring nip portion between the photosensitive drum and the transfer roller in the image forming apparatus in accordance with the first embodiment of the present invention; 
     FIG. 4 is a diagram showing a method of measuring a resistance of the transfer roller; 
     FIG. 5 is a schematic structural diagram showing the detection of a sheet feeding port in accordance with the first embodiment of the present invention; 
     FIG. 6 is a graph showing a change in a transferring current value at a leading end of the transferring material when the transferring materials different in resistance is conveyed; 
     FIG. 7 is a flowchart showing a transferring bias changing control in accordance with a second embodiment of the present invention; 
     FIGS. 8A and 8B are diagrams for explanation of a paper interval and the occurrence of a rear end memory, respectively; 
     FIG. 9 is a flowchart showing a transferring bias changing control in accordance with a third embodiment of the present invention; 
     FIG. 10 is a flowchart showing a transferring bias changing control in accordance with a fourth embodiment of the present invention; 
     FIG. 11 is a schematic structural diagram showing an image forming apparatus; 
     FIG. 12 is a diagram for explanation of an image forming area and margins of the transferring material; 
     FIG. 13 is a diagram for explanation of the image forming area of the transferring material and a position at which the transferring bias is changed; and 
     FIG. 14 is a diagram showing the structure of a top sensor. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Now, a description will be given in more detail of the preferred embodiments of the present invention with reference to the accompanying drawings. The overall construction is the same as that of FIG. 11 so that the explanation thereof will be omitted. 
     (First Embodiment) 
     FIG. 1 is a schematic structural diagram showing a transferring bias control system of an image forming apparatus in accordance with a first embodiment of the present invention, FIG. 2 is a side view showing the vicinity of a transferring nip portion between a photosensitive drum and a transfer roller, and FIG. 3 is a front view showing the vicinity of the transferring nip portion between the photosensitive drum and the transfer roller. 
     In an image forming apparatus according to this embodiment, a transferring bias control is conducted by a PTVC control method which will be described below. 
     In the PTVC control method, when a pass signal of the transferring material P which is conveyed to the transferring nip portion N between the photosensitive drum  1  and the transfer roller  5  is inputted to a controller (DC controller)  30  from the top sensor  11 , the controller  30  outputs a PWM signal having a pulse width corresponding to a desired transferring output voltage to a low pass filter  31 . The pulse width of the PWM signal is stored in a transferring output table (not shown) within the controller  30  in advance. 
     The PWM signal is converted into d.c. by the low pass filter  31  and then amplified by an amplifier (AMP)  32  into the transferring output voltage Vt before being inputted to the high-voltage power supply  33  for transferring. The high-voltage power supply for transferring  33  applies the transferring voltage Vtr to the transfer roller  5  on the basis of the inputted transferring output voltage Vt. A current value  1 t that flows in this state is detected by a current detection circuit  34 , and a signal corresponding to the current value It is inputted to the controller  30  through an A/D convertor  35 . 
     Then, in the case where the constant-voltage control is intended to be conducted, a PWM signal having the pulse width corresponding to a desired voltage value is outputted in accordance with a judgement from the table representative of relation of the PWM signal and the transferring output which is set within the controller  30  in advance. 
     Also, in the case where the constant-current control is intended to be conducted, the pulse width of the PWM signal from the controller  30  is gradually increased continuously until the signal inputted to the controller  30  becomes a value corresponding to a desired current value (a constant current value), and thereafter a voltage (pulse width) is allowed to follow a change in the current value to conduct the constant-current control. 
     Subsequently, the transferring bias control in this embodiment will be described. 
     At the time when the photosensitive drum  1  has been completely charged upon receiving an image forming signal (print signal) from a host computer (not shown), a PTVC detection is executed once in a state where the photosensitive drum  1  and the transfer roller  5  are directly abutted against each other. The PTVC detection is held within the controller  30  with the voltage value as Vto, which is obtained when the transferring current reaches a predetermined constant current value after the output voltage from the high-voltage power supply for transferring  33  is gradually raised. The transferring voltage Vtr applied at the time of transferring is decided on the basis of the detected voltage value Vto and the following transferring control expression which is stored in the controller  30  in advance. 
     
       
           Vtr=αVto+β   (1) 
       
     
     where Vto in the above expression (1) is a generated voltage generated when a driven detected current flows in the transfer roller  5  at the time of the PTVC detection, and α and β are constants arbitrarily decided in accordance with the transferring system. 
     After the decision of the transferring voltage Vtr, the printing operation starts at the time when a preparation for image formation has been completed, and the transferring material P is fed to the transferring nip portion N between the photosensitive drum  1  and the transfer roller  5  in synchronism with the toner image on the photosensitive drum  1 . The synchronization of the transferring material P with the toner image on the photosensitive drum  1  is conducted by timer-count after the transferring material P passes through the top sensor  11 , and the transferring voltage Vtr is constant-voltage applied to conduct the transferring operation simultaneously when the leading end of the transferring material P enters the transferring nip portion N. 
     Also, after the transferring material P has passed through the top sensor  11 , the timer-count again starts, and a time at which the rear end of the transferring material P reaches the transferring nip portion N is reckoned backward. Then, the transferring bias Vtr is changed to a low transferring bias (paper-interval bias) Vlow which is applied at an interval (hereinafter referred to as “paper interval”) between first and second conveyed transferring materials P a given period of time before the rear end of the transferring material P passes through the transferring nip portion N. 
     For example, assuming that a distance between the top sensor  11  and the transferring nip portion N is D (mm) and a process speed is S (mm/sec), a period of time T until the rear end of the transferring material P reaches the transferring nip portion N since it passes through the top sensor  11  is T=D/S (sec), and in the case where the transferring bias is intended to be changed to the low transferring bias at a position of c (mm) from the rear end of the transferring material P, the transferring bias is changed to the low transferring bias (D−c)/S (sec) after the rear end of the transferring material P passes through the top sensor  11 . 
     In this embodiment, the transfer roller  5  is formed of a solid roller (packed texture) having a rubber such as EPDM, silicone, NBR or urethane formed around a mandrel  5   a  made of iron, SUS or the like, or a rubber roller having a foaming-sponge-like intermediate-resistant elastic layer  5   b , as shown in FIGS. 2 and 3, and is in a range of 25 to 70 degrees in roller hardness (at the time of Asker-C/1 kg weight) and in a range of 10 6  to 10 10  Ω in resistance. The elastic layer  5   b  of the transfer roller  5  is secondarily vulcanized after first vulcanization and thereafter the surface of the elastic layer  5   b  is polished to make the outer diameter configuration in a desired dimension. 
     The transfer roller  5  used in this embodiment is formed of an electrically conductive and elastic roller manufactured in such a manner that the elastic layer (intermediate-resistant elastic layer)  5   b  made of an ion electrically conductive solid rubber of NBR which is 8×10 7  Ω in resistance is formed on the mandrel  5   a  which is made of Fe and 6 mm in diameter, and the roller hardness is set to 60 degrees (at the time of Asker-C/1000 g weight), the outer diameter is set to 16 mm, and the longitudinal dimension is set to 218 mm. 
     Also, the transfer roller  5  is brought in pressure contact with the photosensitive drum  1  by a pressure force F which is transmitted from both of longitudinal end portions of the mandrel  5   a  through a bearing  5   c  by a pressure spring  5   d , to thereby form the transferring nip portion N. In this embodiment, the transfer roller  5  is brought in pressure contact with the photosensitive drum  1  by a total pressure 1 Kg. 
     FIG. 4 is a diagram showing a method of measuring a resistance of the transfer roller  5 . 
     As shown in FIG. 4, the transfer roller  5  is abutted against an aluminum cylinder under a total pressure of 1000 g (500 g at one side) and rotated, and the maximum value and the minimum value of voltage values developed between both ends of a resistor  42  when an arbitrary voltage (for example, +2.0 KV) is applied to the mandrel  5   a  from a d.c. high voltage power supply  41  is read by a voltmeter  43 . A mean value of the voltage values applied to the circuit is obtained from the read voltage value to calculate the resistance of the transfer roller  5 . The measuring environments are that the temperature is 20° C. and the humidity is 60%. 
     Subsequently, the change of the transferring bias (changing from the transferring bias to the low transferring bias) under the transferring bias control in this embodiment will be described. 
     In this embodiment, the changing position of the transferring bias at the rear end of the transferring material P is decided in accordance with information as to one-side (front surface) print or double-sided (both surfaces) print (hereinafter referred to as “paper feed port information”) with respect to the transferring material P which is conveyed to the transferring nip portion N. 
     As described above, the rear-end memory image remarkably occurs in the transferring material P which is low in resistance where a current is liable to concentratedly flow at the rear end of the transferring material P. Reversely, the scattering of the toner image at the rear end of the transferring material P is liable to occur in a state where the resistance of the transferring material P is high. Also, in case of the double-sided print (duplex image formation), the rear end memory image is liable to occur during the first printing whereas the toner image is liable to be scattered during the second printing. 
     The image forming apparatus according to this embodiment is structured so that the paper feed intervals are different depending on the designation of the sizes of the transferring material P. The rear end memory does not occur when the paper feed interval of the transferring material P becomes equal to or longer than one round length of the photosensitive drum  1  because a potential produced by a current flowing on the rear end of the transferring material P is attenuated. That is, the rear end memory occurs only when the paper feed interval is the one round length or shorter than the photosensitive drum  1 . 
     In the image forming apparatus according to this embodiment, as shown in FIG. 5, the paper feed port information is detected by the pre-feed sensor  9  and the sheet re-feeding sensor  20 , and the controller  30  judges that the first surface (the front surface) print is executed when the conveyance of the transferring material P is detected by the pre-feed sensor  9  and the second surface (the back surface) print in the duplex image forming operation is executed when the conveyance of the transferring material P is detected by the sheet re-feeding sensor  20 . 
     Also, in the image forming apparatus according to this embodiment, the process speed is 100 mm/sec, A4/16 ppm, the paper interval at the time of continuous paper feeding is 50 mm, and a distance between the top sensor  11  and the transferring nip portion N is 75 mm. The transferring bias control is conducted by the above-described PTVC control, the transferring current that flows at the time of printing the first surface is about 6 μA, the transferring current that flows at the time of printing the second surface in the duplex image forming operation is about 5 μA, and the low transferring bias during the paper interval is 3 μA at the time of paper non-feeding under the control. 
     Then, in this embodiment, the transferring bias changing timing of from the transferring bias to the low transferring bias is set in accordance with the paper feed port information as shown in Table 1. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
             
               
                   
                 Paper Feed Port 
                 Bias Changing Timing 
               
               
                   
                 MPT/Casette 
                 Inside From Rear End 5 mm 
               
               
                   
                 Automatic Double-sided Paper 
                 Inside From Rear End 2 mm 
               
               
                   
                 Feed 
               
               
                   
                   
               
             
          
         
       
     
     In the first-surface printing (in the case where the paper feed port is a multi-purpose tray (MPT) and a cassette paper feed), in order to suppress the occurrence of the rear end memory, the changing position (timing) of the transferring bias is set to 5 mm inside of the rear end of the transferring material P (the bias is changed 700 msec after the rear end of the transferring material P passes through the top sensor  11 ). Conversely, in the second-surface printing (automatic double-sided paper feed), in order to prevent the scattering of the toner image, the changing position of the transferring bias is set to 2 mm inside of the rear end of the transferring material P (the bias is changed 730 msec after the rear end of the transferring material P passes through the top sensor  11 ). The multi-purpose tray is a tray attached to the exterior. 
     Then, under the conditions where the rear end margin of the transferring material P is 5 mm in a normal humidity (humidity of 60%), at the time of one side (front surface) printing operation and automatic double-side (duplex) printing operation on a plain A4 size paper (75 g/m 2 ), the occurrence of the rear end memory and the occurrence of the scattering of the toner image (in this embodiment, the scattering of the toner image is called “image rear end explosion” or merely “explosion”) were evaluated. Table 2 shows the evaluated results. In Table 2, comparative examples to this embodiment (1 in the figure) show examples in which the transferring bias was changed at a given timing regardless of the paper feed port. A comparative example 1 shows a case in which the transferring bias is changed at 2 mm inside of the rear end of the transferring material P whereas a comparative example 2 shows a case in which the transferring bias is changed at 5 mm inside of the rear end of the transferring material P. 
     
       
         
               
               
               
               
             
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                   
                 Rear End 
               
               
                   
                   
                 Rear End Memory 
                 Explosion 
               
               
                   
                   
                 Frequency of 
                 Frequency of 
               
               
                   
                 Print Side 
                 Occurrence 
                 Occurence 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                 Example 1 
                 One Side Print 
                 0/10 
                 0/10 
               
               
                   
                 Automatic 
               
               
                   
                 Double-sided 
                 0/10 
                 0/10 
               
               
                   
                 Print 
               
               
                 Comparative 
                 One Side Print 
                 9/10 
                 0/10 
               
               
                 Example 1 
                 Automatic 
                 0/10 
                 0/10 
               
               
                   
                 Double-sided 
               
               
                   
                 Print 
               
               
                 Comparative 
                 One Side Print 
                 0/10 
                 0/10 
               
               
                 Example 2 
                 Automatic 
                 0/10 
                 5/10 
               
               
                   
                 Double-sided 
               
               
                   
                 Print 
               
               
                   
               
             
          
         
       
     
     In the comparative example 1, because the transferring bias changing point is set to 2 mm inside of the rear end of the transferring material P, the scattering of the toner image (image rear end explosion) does not occur even if the changing position is varied. Conversely, because the transferring bias changing position is in the vicinity of the rear end of the transferring material P, when the transferring material P passes through the transferring nip portion N during the one-side (front surface) printing operation, an excessive current flows with the result that the rear end memory occurs. 
     Also, in the comparative example 2, because the transferring bias changing point is set to 5 mm inside of the rear end of the transferring material P, the rear end memory does not occur even if the changing position is varied. However, if the transferring bias changing position is varied, because a voltage drops from the transferring bias Vtr to the low transferring bias Vlow within the image forming area (printing area) of the transferring material P, the transferring current is short during the second-surface printing operation where the resistance of the transferring material (paper) P is high with the result that the scattering of the toner image (image rear end explosion) occurs. 
     On the other hand, in this embodiment (embodiment 1), during the one-side (front surface) printing operation and at the time of printing the first surface during the double-sided (duplex) printing operation, where the rear end memory is liable to occur and the occurrence of scattering of the toner image (image rear end explosion) is difficult to occur, the bias changing is conducted at 5 mm inside of the rear end of the transferring material P. Then, at the time of printing the second-surface (back surface) during the double-sided (duplex) printing operation where because the resistance of the transferring material P is high, the current is concentrated at the rear end of the transferring material P and difficult to flow, and the rear end memory is difficult to occur, the transferring bias changing is conducted at 2 mm inside of the rear end of the transferring material P. As a result, in any of the cases, there are obtained an excellent image where both of the rear end memory and the scattering of the toner image (image rear end explosion) do not occur. 
     As described above, in this embodiment, since the changing position of the transferring bias at the rear end of the transferring material P is changed in accordance with the paper feed port (the one-side (front surface) print or the double-sided (duplex) print with respect to the transferring material P), the rear end memory and the scattering of the toner image (image rear end explosion) are prevented, thereby being capable of obtaining an excellent image. 
     (Second Embodiment) 
     A second embodiment will be described with reference to the image forming apparatus in the first embodiment. In the first embodiment, the changing position of the transferring bias at the rear end of the transferring material P is changed in accordance with the paper feed port (the one-side (front surface) print or the double-sided (duplex) print with respect to the transferring material P) information. On the other hand, in the second embodiment, the changing position of the transferring bias at the rear end of the transferring material P is changed in accordance with the resistance of the transferring material P and the resistance of the transfer roller  5 . Other structures and image forming operation are identical with those in the first embodiment. 
     It has been found that the rear end memory and the scattering of the toner image described above are phenomenons occurring depending on the resistance of the transferring material P. 
     As described above, the rear end memory is caused by a current which concentratedly flows at the rear end of the transferring material P with respect to the photosensitive drum  1 . The current that causes the rear end memory to occur is more liable to flow as the resistance of the transferring material P is lower, and the rear end memory is liable to occur. This phenomenon remarkably occurs particularly under the circumstances of a high humidity where the transferring material P absorbs moisture and its resistance is low. 
     On the other hand, the scattering of the toner image is liable to occur when the resistance of the transferring material P is high. This is because the unevenness of the transferred charges on the transferring material P which is produced during the transferring operation is not averaged at the transferring material P high in resistance due to the presence/absence of toner. In particular, for the above reason, the scattering of the toner image is liable to occur at the time of printing the second surface (back surface) during the double-sided (duplex) printing operation. 
     Under the above circumstance, in this embodiment, the transferring current that flows through the transferring material P during the transferring operation is detected to estimate the resistance of the transferring material P. If the resistance is high, the transferring bias changing position is set to be late whereas if the resistance is low, the transferring bias changing position is set to be early. 
     More specifically, in FIG. 1, the current value that flows in the photosensitive drum  1  through the transferring material P from the transfer roller  5  during the transferring operation is detected by the current detection circuit  34 , and converted into a digital value by the A/D convertor  35 . The digital value is inputted to the controller  30 , and the resistance of the transferring material P is estimated from the current value that flows in the transferring roller  5  by the controller  30 . In this situation, in order to more accurately detect the resistance of the transferring material P, in this embodiment, the above current detection is conducted in the margin area at the leading end of the transferring material P in the transferring material conveying direction when the transferring material P enters the transferring nip portion N, to thereby estimate the resistance of the transferring material P. 
     FIG. 6 is a graph showing a change in a transferring current value at a leading end of the transferring material P when the transferring materials P different in resistance is conveyed to the transferring nip portion N. The axis of ordinate is a time t whereas the axis of abscissa is a transferring current value I. 
     In this embodiment, the above-described transferring voltage Vtr at the time when the paper passes through the transferring nip portion N which is decided on the basis of the detected results of the PTVC is changed from the voltage Vto and then applied as a constant voltage at the leading end of the transferring material P. The transferring current that flows during the transferring operation changes depending on the resistance of the transferring material P. For that reason, as shown in FIG. 8, the transferring current value is large as indicated by a line A in a case where the transferring material P is low in resistance, and conversely the transferring current value becomes larger as indicated by a line C as the resistance of the transferring material P becomes higher. A line N is a transferring current value in the case where the resistance of the transferring material P is intermediate. 
     Also, the rear end memory is liable to occur because the current which concentratedly flows at the rear end of the transferring material P becomes larger as the transferring current value is larger, as described above. Conversely, the scattering of the toner image remarkably occurs because the transferring current becomes shorter at the transferring voltage changing position of the rear end of the transferring material P as the transferring current value is smaller. 
     Under the above circumstances, in this embodiment, the transferring current value while the leading end of the transferring material P passes through the transferring nip portion N is monitored by the controller  30  for a time T 1 , and the states of the transferring material P (the resistance, the hygroscopic state and the print side of the transferring material P) are totalized in accordance with the magnitude of the transferring current at that time, and then detected on the basis of the transferring current value. The controller  30  changes the transferring voltage changing position at the rear end of the transferring material P on the basis of the transferring current value thus detected. 
     Then, there were evaluated the transferring current value, the occurrence of the rear end memory and the occurrence of the toner image scatter (image rear end explosion) when the transferring materials different in resistance (low-resistant transferring material, plain paper and high-resistant transferring material) are subjected to one-side (front surface) print and automatic double-sided (duplex) print under the conditions of normal humidity (humidity of 60%), low humidity (humidity of 10%) and high humidity (humidity of 85%). Table 3 shows the evaluated results. The image forming apparatus and the transferring conditions in the evaluation are identical with those in the first embodiment. 
     The above results are exhibited in a case where the detection of the transferring current value is conducted at a portion where the leading end of the transferring material enters the transferring nip portion N by 4 mm, and the transferring bias changing position at the rear end of the transferring material P is 0 mm, 2 mm, 4 mm and 6 mm inside of the rear end of the transferring material P, respectively. Also, in this embodiment, a thin paper A4 in size (60 g/m 2 ) is used as a low-resistant paper, a recycled paper A4 is used as a high-resistant paper, and a transferring material A4 in size and 75 g/m 2  is used as a plain paper. 
     As is apparent from the evaluated results shown in Table 3, the transferring current value is higher as the humidity is higher because the transferring material P absorbs the moisture more, the transferring current value is lower as the humidity is lower because of no influence of humidity, and the transferring current value is an intermediate value therebetween in the normal humidity. 
     
       
         
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 3 
               
             
             
               
                   
                   
               
               
                   
                 Transferring 
                 0 mm 
                 2 mm 
                 4 mm 
                 6 mm 
               
             
          
           
               
                   
                 Transferring 
                   
                 Current 
                 Rear End 
                   
                 Rear End 
                   
                 Rear End 
                   
                 Rear End 
                   
               
               
                 Environment 
                 Material 
                 Print Side 
                 Value 
                 Memory 
                 Explosion 
                 Memory 
                 Explosion 
                 Memory 
                 Explosion 
                 Memory 
                 Explosion 
               
               
                   
               
               
                 High 
                 Low 
                 One Side 
                 8.5 μA 
                 × 
                 ◯ 
                 × 
                 ◯ 
                 Δ 
                 ◯ 
                 ◯ 
                 ◯ 
               
               
                 Humidity 
                 Resistance 
                 Double- 
                 5.5 μA 
                 × 
                 ◯ 
                 Δ 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
                 Δ× 
               
               
                   
                   
                 Side 
               
               
                   
                 Plain Paper 
                 One Side 
                   8 μA 
                 × 
                 ◯ 
                 × 
                 ◯ 
                 Δ 
                 ◯ 
                 ◯ 
                 ◯ 
               
               
                   
                   
                 Double- 
                 6.5 μA 
                 × 
                 ◯ 
                 Δ 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
                 Δ◯ 
               
               
                   
                   
                 Side 
               
               
                   
                 High 
                 One Side 
                   6 μA 
                 × 
                 ◯ 
                 Δ 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
                 Δ 
               
               
                   
                 Resistance 
                 Double- 
                   4 μA 
                 Δ 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
                 Δ 
                 ◯ 
                 × 
               
               
                   
                   
                 Side 
               
               
                 Normal 
                 Low 
                 One Side 
                   7 μA 
                 × 
                 ◯ 
                 Δ× 
                 ◯ 
                 Δ 
                 ◯ 
                 ◯ 
                 ◯ 
               
               
                 Humidity 
                 Resistance 
                 Double- 
                 4.5 μA 
                 Δ 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
                 Δ 
                 ◯ 
                 × 
               
               
                   
                   
                 Side 
               
               
                   
                 Plain Paper 
                 One Side 
                   6 μA 
                 × 
                 ◯ 
                 Δ 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
                 Δ 
               
               
                   
                   
                 Double- 
                 4.5 μA 
                 Δ 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
                 Δ 
                 ◯ 
                 × 
               
               
                   
                   
                 Side 
               
               
                   
                 High 
                 One Side 
                   4 μA 
                 Δ 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
                 Δ 
                 ◯ 
                 × 
               
               
                   
                 Resistance 
                 Double- 
                 3.5 μA 
                 Δ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
                 × 
                 ◯ 
                 × 
               
               
                   
                   
                 Side 
               
               
                 Low 
                 Low 
                 One Side 
                   6 μA 
                 × 
                 ◯ 
                 Δ 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
                 Δ 
               
               
                 Humidity 
                 Resistance 
                 Double- 
                   4 μA 
                 Δ 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
                 Δ 
                 ◯ 
                 × 
               
               
                   
                   
                 Side 
               
               
                   
                 Plain Paper 
                 One Side 
                   5 μA 
                 × 
                 ◯ 
                 Δ 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
                 Δ× 
               
               
                   
                   
                 Double- 
                   4 μA 
                 Δ 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
                 Δ 
                 ◯ 
                 × 
               
               
                   
                   
                 Side 
               
               
                   
                 High 
                 One Side 
                 3.5 μA 
                 Δ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
                 ◯ 
                 × 
                 ◯ 
                 × 
               
               
                   
                 Resistance 
                 Double- 
                   3 μA 
                 ◯ 
                 ◯ 
                 ◯ 
                 Δ 
                 ◯ 
                 × 
                 ◯ 
                 × 
               
               
                   
                   
                 Side 
               
               
                   
               
               
                 ◯ : Excellent  
               
               
                 Δ: Good  
               
               
                 ×: No-good  
               
             
          
         
       
     
     Then, an excellent image was obtained from the evaluated results shown in Table 3 by conducting the changing of the transferring bias from the transferring bias Vtr to the low transferring bias Vto at 0 mm from the rear end of the transferring material P, if the transferring current value is 3 μA or less as shown in Table 4, because the rear end memory does not occur and the scattering of the toner image (image rear end explosion) is liable to occur. 
     
       
         
               
               
               
               
               
             
           
               
                   
                 TABLE 4 
               
               
                   
                   
               
               
                   
                 Transferring 
                   
                   
                   
               
               
                   
                 Current 
                 Rear End 
                 Rear End 
                 Rear End 
               
               
                   
                 Value 
                 Memory 
                 Explosion 
                 Off Timing 
               
               
                   
                   
               
             
             
               
                   
                 3 μA or less 
                 Exellent 
                 Excellent 
                 0 mm 
               
               
                   
                   
                   
                 if within 
               
               
                   
                   
                   
                 2 mm 
               
               
                   
                 From 3.5 to 
                 Excellent 
                 Excellent 
                 Insdie 2 mm 
               
               
                   
                 5 μA 
                 if 2 mm 
                 if within 
               
               
                   
                   
                 or more 
                 2 mm 
               
               
                   
                 From 5.5 to 
                 Excellent 
                 Excellent 
                 Inside 4 mm 
               
               
                   
                 6.5 μA 
                 if 4 mm 
                 if within 
               
               
                   
                   
                 or more 
                 4 mm 
               
               
                   
                 7 μA or more 
                 Excellent 
                 Excellet 
                 Inside 6 mm 
               
               
                   
                   
                 if 6 mm 
               
               
                   
                   
                 or more 
               
               
                   
                   
               
             
          
         
       
     
     Also, because the rear end memory occurs more as the transferring current value increases more, and the scattering of the toner image (image rear end explosion) is less liable to occur, the optimum position of the transferring bias changing gradually enters inside from the rear end of the transferring material P. An excellent image was obtained by changing from the transferring bias Vtr to the low transferring bias Vto at 2 mm inside of the rear end of the transferring material P if the transferring current value is 3.5 to 5 μA, at 4 mm inside of the rear end of the transferring material P if the transferring current value is 5.5 to 6.5 μA, and at 6 mm inside of the rear end of the transferring material P if the transferring current value is 7 μA or more. 
     Subsequently, the above-described transferring bias changing control during the transferring operation in accordance with this embodiment will be described with reference to a flowchart shown in FIG.  7 . 
     First, upon receiving a print signal from a host computer (not shown), printing (image forming) starts and paper feeding starts (step S 1 ). Then, if the transferring material P passes through the top sensor  11  and a top signal is outputted (on) (step S 2 ), the controller  30  starts timer-count (step S 3 ), and the transferring bias Vtr is applied at the leading end of the transferring material P in synchronism with an image on the photosensitive drum  1  at the transferring nip portion N (step S 4 ). 
     Then, the transferring current value (AD value) is detected 40 msec after the application of the transferring bias Vtr (corresponding to 4 mm from the leading end of the transferring material P at a process speed of 100 mm/sec) (step S 5 ), and the controller  30  compares the inputted transferring current value (AD value) with a table prepared in advance to decide the transferring bias changing position (step S 6 ). 
     Then, if the rear end of the transferring material P passes through the top sensor  11  to turn off the top signal (step S 7 ), the controller  30  restarts the timer-count (step S 8 ), estimates the rear end of the transferring material P at the transferring nip portion N, and changes from the transferring voltage Vtr to the low transferring bias Vto on the basis of the transferring bias changing position decided in the above manner (step S 9 ). Then, the printing (image forming) operation is terminated, or the next printing (image forming) operation is continuously conducted (step S 10 ). 
     The timer-count based on the top signal from the top sensor  10  slightly fluctuates. However, even if the transferring bias changing position is shifted from a set value because the time slightly fluctuates, the rear end memory and the scattering of the toner image which slightly occur fall within a level that does not cause any problem in practical use, and the results of actually printing the respective transferring materials used at the present time under the respective environments 50 sheets, respectively, were also excellent. 
     As described above, in this embodiment, the transferring current value that flows in the transferring material P is measured at the leading end of the transferring material P, and the transferring bias changing position at the rear end of the transferring material P is changed on the basis of the measurement, thereby being capable of preventing the rear end memory and the scattering of the toner image (image rear end explosion) to obtain an excellent image. 
     (Third Embodiment) 
     A third embodiment will be described with reference to the image forming apparatus in the first embodiment. In the third embodiment, a paper feed interval of the transferring materials P which are fed to the transferring nip portion N is monitored, and the changing position of the transferring bias at the rear end of the transferring material P is changed in accordance with the paper feed port information and the paper interval information. 
     As was previously described, the rear end memory is a phenomenon in which, as shown in FIG. 8A, a memory produced on the photosensitive drum  1  by a current which concentratedly flows at the rear end of a first transferring material P occurs on a second transferring material P as a stripe image after the photosensitive drum  1  rotates one round. As shown in FIG. 8B, the memory on the photosensitive drum  1  is attenuated every time charging is repeated. Normally, the memory substantially disappears when printing is conducted in two rounds of the photosensitive drum  1 , and the rear end memory does not appear on the image if the paper interval (an interval between the rear end of the first transferring material P and the leading end of the second transferring material P) is equal to or longer than one round of the photosensitive drum  1 . 
     Accordingly, in this embodiment, the paper interval is judged from the paper feed interval, and the transferring bias changing position at the rear end of the transferring material P is changed between a case where the paper interval is equal to or longer than one round of the photosensitive drum  1  and a case where the paper interval is shorter than one round thereof. In this embodiment, the process speed of the image forming apparatus is set to 150 mm/sec, and other image forming conditions are identical with those in the first embodiment. 
     Assuming that the paper feed interval is T 1  (sec), the size of the transferring material P (length) is M (mm) and the process speed is S (mm/sec), the paper interval T 2  (sec) is obtained from the following expression. 
     
       
           T   2 = T   1 ( M/S )  (2) 
       
     
     The controller  30  can judge the paper interval on the basis of the above expression (2). 
     Subsequently, the transferring bias changing control in this embodiment will be described with reference to a flowchart shown in FIG.  9 . 
     First, upon receiving a print signal from a host computer (not shown)(step S 11 ), printing starts and paper feeding starts (step S 12 ). In this situation, the controller  33  starts the timer-count of the paper feed interval on the basis of the pass signal of the (first) transferring material P which is inputted from the top sensor  11  (step S 13 ). 
     Then, after the transferring material P is conveyed to the transferring nip portion N through the top sensor  11  in synchronism with an image on the photosensitive drum, if a next (second) transferring material P is conveyed, the paper feed interval between the previous (first) transferring material and the second transferring material is detected by the controller  33  (step S 14 ). 
     The controller  33  judges the paper interval between the first and second transferring materials P according to the pass information of the transferring material P from the top sensor  11  (step S 15 ), and if the paper interval is longer than one round of the photosensitive drum  1 , the transferring bias changing position (changing point) at the rear end of the first transferring material P is set to X mm inside of the rear end of the transferring material P (step S 16 ). X (mm) in step S 16  is set to X=0 mm in this embodiment, that is, the rear end of the transferring material P. 
     In step S 16 , if the detected paper interval is shorter than one round of the photosensitive drum  1 , the controller  33  judges from the inputted paper feed port information whether it is the one-side print (front surface print) or the double-sided print (back surface print in the duplex printing operation) (step S 17 ). In case of the one-side print (front surface print) where the transferring material P is fed from the paper feed roller  8 , the transferring bias changing position (point) is set to X mm inside of the rear end of the transferring material P (step S 18 ). X (mm) in step S 18  is set to 6 mm inside of the rear end of the transferring material P in this embodiment. 
     In step S 17 , in case of the double-sided print (back surface print in the duplex printing operation) where the transferring material P is fed from the paper re-feed roller  20 , the transferring bias changing position (point) is set to Y mm inside of the rear end of the transferring material P (X&gt;Y) (step S 19 ). Y (mm) in step S 19  is set to 2 mm inside of the rear end of the transferring material P in this embodiment. 
     Then, the transferring bias Vtr is changed to the low transferring bias (paper interval bias) Vlow in accordance with the transferring bias changing position set in steps S 16 , S 18  and S 19 , respectively (step S 20 ). Then, the printing (image forming) operation is terminated, or next printing (image forming) operation is continuously conducted (step S 21 ). 
     As described above, in this embodiment, since the paper interval is calculated from the paper feed interval, and the transferring bias changing position is changed in accordance with the paper feed port information and the paper interval information, an excellent image can be obtained by preventing the rear end memory and the scattering of the toner even in the image forming apparatus high in process speed which make it difficult to adjust the transferring bias changing position. 
     (Fourth Embodiment) 
     A fourth embodiment will be described with reference to the image forming apparatus in the first embodiment. In the fourth embodiment, the changing position of the transferring bias at the rear end of the transferring material P is changed in accordance with the above-described paper feed port information and resistance of the transfer roller  5 . In this embodiment, the process speed of the image forming apparatus is set to 150 mm/sec, and other image forming conditions are identical with those in the first embodiment. 
     In the first embodiment, in order to prevent both of the occurrences of the rear end memory and the toner scattering, the transferring bias changing position is set to inside of the rear end of the transferring material P only at the time of the one-side print (front surface print). However, in the environments of the low temperature and low humidity where the resistance of the transfer roller  5  becomes high, the transferring bias Vtr becomes a high voltage that exceeds 3 KV. If the transferring bias Vtr becomes high, when the transferring bias changes from the transferring bias Vtr to the low transferring bias Vlow, a predetermined period of time is required until the voltage becomes completely the low transferring bias Vlow due to the characteristics of a high-voltage circuit in the high-voltage power supply  33  for transferring. 
     For that reason, in the case where the transferring bias Vtr is high, in order to prevent the rear end memory, it is necessary that the transferring bias changing position is set to more inside of the rear end of the transferring material P than that in the first embodiment. 
     Subsequently, the transferring bias changing control in this embodiment will be described with reference to a flowchart shown in FIG.  10 . 
     First, upon receiving a print signal from a host computer (not shown), printing (image formation) starts and paper feeding starts (step S 31 ). In this situation, the controller  33  judges from the inputted paper feed port information whether it is the one-side print (front surface print) or the double-sided print (back surface print in the duplex printing operation) (step S 32 ). In case of the double-sided print (back surface print during the double-sided printing operation) where the transferring material P is fed from the paper re-feed roller  20 , in order to prevent the scattering of toner, the transferring bias changing position (point) is set to a mm inside of the rear end of the transferring material P (step S 33 ). 
     Also, in step S 32 , at the time of the one-side print (front surface print) where the transferring material P is fed from the paper feed roller  8 , if the detected value of PTVC is smaller than a predetermined value on the basis of the detected result of PTVC, it is judged that the transfer roller  5  is relatively low in resistance, and the transferring bias changing position (point) is set to b (a&lt;b) mm inside of the rear end of the transferring material P (steps S 34  and S 35 ). 
     Also, in step S 34 , if the detected value of PTVC is equal to or larger than the predetermined value on the basis of the detected result of PTVC, it is judged that the transfer roller  5  is high in resistance, and the applied transferring bias is also high, the transferring bias changing position (point) is set to c (a&lt;b&lt;c) mm inside of the rear end of the transferring material P (step S 36 ). In this embodiment, a is set to 2 (mm), b is set to 5 (mm) and c is set to 7 (mm). 
     After the transferring bias changing position has been decided in steps S 33 , S 35  and S 36 , when the rear end of the transferring material P passes through the top sensor  11  to turn off the top signal (step S 37 ), the controller  30  starts the timer-count (step S 38 ), estimates the rear end of the transferring material P at the transferring nip portion N, and changes from the transferring voltage Vtr to the low transferring bias Vlow on the basis of the transferring bias changing position decided in the above manner (step S 39 ). Then, the printing (image forming) operation is terminated, or the next printing (image forming) operation is continuously conducted (step S 40 ). 
     As described above, in this embodiment, since the transferring bias changing position at the rear end of the transferring material P is changed in accordance with the paper feed port information and the resistance of the transfer roller  5 , an excellent image can be obtained by preventing the rear end memory even in the case where the transfer bias is a high voltage. 
     The above description was given of the embodiments of the present invention, however, the present invention is not limited to those embodiments and can be modified within the technical concept of the invention. 
     The foregoing description of the preferred embodiments of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and modifications and variations are possible in light of the above teachings or may be acquired from practice of the invention. The embodiments were chosen and described in order to explain the principles of the invention and its practical application to enable one skilled in the art to utilize the invention in various embodiments and with various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the claims appended hereto, and their equivalents.