Patent Publication Number: US-8121506-B2

Title: Image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2009-022752 filed on Feb. 3, 2009. 
     BACKGROUND 
     Technical Field 
     The present invention relates to an image forming apparatus. 
     SUMMARY 
     A first aspect of the present invention is an image forming apparatus including: an image carrier that carries a developed image that has been developed on a surface thereof using a developer; a transfer member that transfers the developed image from the image carrier to a belt-shaped member; an electric power supply unit that supplies electric power to the transfer member; a measurement unit that measures a combined resistance value of the image carrier, the belt-shaped member, and the transfer member; and a controller that controls the electric power supply unit such that the electric power supplied to the transfer member is changed from a predetermined first supply value to a second supply value, the second supply value being larger than the first supply value, when the combined resistance value measured by the measurement unit is lower than a predetermined combined resistance value. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  is a side view illustrating a schematic configuration of a main part of an image forming apparatus according to an exemplary embodiment of the invention; 
         FIG. 2  is a block diagram illustrating a configuration of a main part of an electric system in the image forming apparatus of the exemplary embodiment; 
         FIG. 3  is a flowchart illustrating a flow of a transfer defect suppressing process program of the exemplary embodiment; and 
         FIG. 4  is a graph illustrating an example of a relationship of an accumulative transfer amount of toner image transferred from a photoreceptor drum to an intermediate transfer belt from the beginning of a first electric power control to a transfer power supply output level in a first electric power control, or to a transfer power supply output level in a second electric power control. 
     
    
    
     DETAILED DESCRIPTION 
     An exemplary embodiment of the invention will be described below with reference to the drawings. 
       FIG. 1  is a side view illustrating a schematic configuration of a main part of an image forming apparatus  10  according to an exemplary embodiment of the invention. 
     Referring to  FIG. 1 , the image forming apparatus  10  includes a photoreceptor drum  12  that is rotated in a direction of an arrow A, i.e., in a slow scanning direction at a predetermined rotating speed by a motor (not illustrated). A charger  14  is provided in an outer circumferential surface of the photoreceptor drum  12  while being in contact with the outer circumferential surface. The charger  14  charges the outer circumferential surface. 
     A laser beam scanning device  16  is disposed further downstream side of the photoreceptor drum  12  in the direction of the arrow A than the charger  14 . The laser beam scanning device  16  modulates a laser beam emitted from a light source according to an image to be formed, and the laser beam scanning device  16  deflects the laser beam in a fast scanning direction to scan the outer circumferential surface of the photoreceptor drum  12  in parallel with an axis line of the photoreceptor drum  12 , thereby forming an electrostatic latent image on the outer circumferential surface of the photoreceptor drum  12 . 
     A development device  18  is disposed further downstream side of the photoreceptor drum  12  in the direction of the arrow A than the laser beam scanning device  16 . The development device  18  includes a roller-shaped storage body that is rotatably disposed. Four storage portions corresponding to yellow (Y), magenta (M), cyan (C), and black (K) colors are formed in the storage body, and development sections  18 Y,  18 M,  18 C, and  18 K are provided in the storage portions. The development sections  18 Y,  18 M,  18 C, and  18 K include development rollers (not illustrated), and Y, M, C, and K color toners are stored in the development sections  18 Y,  18 M,  18 C, and  18 K. An erasing and cleaning device  22  is disposed on the opposite side of the photoreceptor drum  12  to the development device  18 . The erasing and cleaning device  22  has a function of erasing electricity in the outer circumferential surface of the photoreceptor drum  12  and a function of removing unnecessary toner remaining on the outer circumferential surface thereof. 
     In the image forming apparatus  10  of the exemplary embodiment, the color image is formed while the photoreceptor drum  12  is rotated four revolutions. That is, in a period during which the photoreceptor drum  12  is rotated four revolutions, the charger  14  continuously charges the outer circumferential surface of the photoreceptor drum  12 , the erasing and cleaning device  22  continuously erases (removes) the electricity from the outer circumferential surface, and the laser beam scanning device  16  repeatedly scans the outer circumferential surface of the photoreceptor drum  12  with the laser beam that is modulated according to one of the Y, M, C, and K items of image information expressing the color image to be formed while switching the image information used for the modulation of the laser beam every one revolution of the photoreceptor drum  12 . While the development roller of one of the development sections  18 Y,  18 M,  18 C, and  18 K is in contact with the outer circumferential surface of the photoreceptor drum  12 , the development device  18  actuates the development section that is in contact with the outer circumferential surface to develop the electrostatic latent image formed on the outer circumferential surface of the photoreceptor drum  12  with a specific color, and the development device  18  forms a specific toner image on the outer circumferential surface of the photoreceptor drum  12 . The development device  18  repeats the image formation while the development section used in the development of the electrostatic latent image is switched every one revolution of the photoreceptor drum  12 . 
     Therefore, the Y, M, C, and K toner images are sequentially formed on the outer circumferential surface of the photoreceptor drum  12  while superimposed on one another every revolution of the photoreceptor drum  12 , and the color toner image is formed on the outer circumferential surface of the photoreceptor drum  12  after the photoreceptor drum  12  is rotated four revolutions. 
     An endless intermediate transfer belt  20  is provided below the photoreceptor drum  12 . The intermediate transfer belt  20  is entrained about rollers  24 A,  24 B,  24 C, and  24 D, and the endless intermediate transfer belt  20  is disposed such that an outer circumferential surface of the endless intermediate transfer belt  20  is in contact with the outer circumferential surface of the photoreceptor drum  12 . The rollers  24 A,  24 B,  24 C, and  24 D are rotated by the torque of a motor (not illustrated) being transmitted and rotate the intermediate transfer belt  20  in a direction of an arrow B. 
     A primary transfer roller  26  is disposed at the opposite side of the intermediate transfer belt  20  to the photoreceptor drum  12 . The primary transfer roller  26  presses the intermediate transfer belt  20  against the outer circumferential surface of the photoreceptor drum  12 . 
     A transfer power supply  28  is provided in the image forming apparatus  10 , and the transfer power supply  28  supplies an electric power to the primary transfer roller  26  in order to transfer the toner image on the photoreceptor drum  12  to the intermediate transfer belt  20 . 
     Accordingly, the transfer power supply  28  supplies the electric power to the primary transfer roller  26 , and the primary transfer roller  26  presses the intermediate transfer belt  20  against the outer circumferential surface of the photoreceptor drum  12 , whereby the toner image formed on the outer circumferential surface of the photoreceptor drum  12  is transferred to an image forming surface of the intermediate transfer belt  20 . When the toner image formed on the outer circumferential surface of the photoreceptor drum  12  is transferred to the intermediate transfer belt  20 , the erasing and cleaning device  22  cleans a region where the transferred toner image is held in the outer circumferential surface of the photoreceptor drum  12 . 
     A sheet storage  30  is disposed below the intermediate transfer belt  20 , and many stacked sheets P that are of a recording medium are stored in the sheet storage  30 . In the drawing, a feed roller  32  is disposed at the upper left on the sheet storage  30 , and pairs of rollers  34  and  36  are disposed at the downstream side in a direction in which the feed roller  32  feeds the sheet P. The uppermost sheet P in the stacked state is fed from the sheet storage  30  by the rotation of the feed roller  32 , and the sheet P is transported by the pairs of rollers  34  and  36 . 
     A secondary transfer roller  38  is disposed at the opposite side of the intermediate transfer belt  20  to the roller  24 A, and the secondary transfer roller  38  presses the intermediate transfer belt  20  against the outer circumferential surface of the roller  24 A. The sheet P transported by the pairs of rollers  34  and  36  is delivered between the intermediate transfer belt  20  and secondary transfer roller  38 , and the secondary transfer roller  38  transfers the toner image formed on the image forming surface of the intermediate transfer belt  20  to the sheet P. As with the primary transfer roller  26 , a transfer electric power is supplied to the secondary transfer roller  38 . 
     A fixing device  40  is disposed further downstream in the sheet transporting direction (direction of an arrow C of  FIG. 1 ) than the secondary transfer roller  38 . The fixing device  40  includes a heating roller  40 A that heats the toner image on the sheet P and a roller  40 B that is pressed against the heating roller  40 A. When the sheet P is passed through a nip part between the heating roller  40 A and the roller  40 B, the toner image is fused and solidified, and therefore the toner image is fixed to the sheet P. Then, a sheet exit roller (not illustrated) transports the sheet P to the outside of the image forming apparatus  10 . The sheet exit roller is disposed further downstream in the sheet transporting direction than the fixing device  40 . 
     The image forming apparatus  10  includes a temperature sensor  42  that measured a temperature in a space of the apparatus and a humidity sensor  44  that measures humidity in the space of the apparatus. In the image forming apparatus  10  of the exemplary embodiment, a thermistor is used as the temperature sensor  42 . Alternatively, other temperature sensors such as a platinum resistance thermometer and a thermocouple may obviously be used as the temperature sensor  42 . In the image forming apparatus  10  of the exemplary embodiment, a polymer-membrane humidity sensor is used as the humidity sensor  44 . Alternatively, other humidity sensors such as a ceramic humidity sensor and an electrolytic humidity sensor may obviously be used as the humidity sensor  44 . 
       FIG. 2  is a block diagram illustrating a configuration of a main part of an electric system in the image forming apparatus  10  of the exemplary embodiment. 
     Referring to  FIG. 2 , the image forming apparatus  10  includes CPU (Central Processing Unit)  60 , ROM (Read Only Memory)  62 , RAM (Random Access Memory)  64 , NVM (Non Volatile Memory)  66 , a UI (User Interface) screen  68 , and a communication interface  70 . 
     CPU  60  controls the whole operation of the image forming apparatus  10 . ROM  62  acts as a storage device in which a control program for controlling actuation of the image forming apparatus  10 , a transfer defect suppressing process program (described later), and various parameters are previously stored. RAM  64  is used as a work area in executing various programs. Various kinds of information that should be retained even if the power of the apparatus is turned off are stored in NVM  66 . 
     The UI screen  68  includes a touch screen display or the like in which a transparent touch screen is laminated on a display. Various kinds of information are displayed on a display surface of the display, and a user can input desired information or a desired instruction by touching the touch screen display. 
     The communication interface  70  is connected to a terminal device  72  such as a personal computer, and the communication interface  70  receives various kinds of information such as image information expressing the image formed on the sheet P from the terminal device  72 . 
     CPU  60 , ROM  62 , RAM  64 , NVM  66 , the UI screen  68 , and the communication interface  70  are connected to one another through a system bus BUS. Accordingly, CPU  60  accesses ROM  62 , RAM  64 , and NVM  66 , displays various kinds of information on the UI screen  68 , obtains contents of an instruction provided at the UI screen  68  by the user, and receives various kinds of information from the terminal device  72  through the communication interface  70 . 
     The image forming apparatus  10  includes an image forming engine unit  74  that forms the image on the sheet P by a xerographic system. The image forming engine unit  74  includes the photoreceptor drum  12 , the charger  14 , the laser beam scanning device  16 , the development device  18 , the erasing and cleaning device  22 , the rollers  24 A,  24 B,  24 C, and  24 D, the primary transfer roller  26 , the transfer power supply  28 , the pairs of rollers  34  and  36 , the secondary transfer roller  38 , the fixing device  40 , and the motors (not illustrated) that drive the rollers. 
     The image forming engine unit  74  is also connected to the system bus BUS. Accordingly, CPU  60  controls the actuation of the image forming engine unit  74 . 
     The image forming apparatus  10  includes a resistance value measuring unit  76  and a heating roller temperature measuring unit  78 . The resistance value measuring unit  76  measures a combined resistance value of the photoreceptor drum  12 , the intermediate transfer belt  20 , and the primary transfer roller  26 . The heating roller temperature measuring unit  78  measures a temperature of the heating roller  40 A. The resistance value measuring unit  76  and the heating roller temperature measuring unit  78  are also connected to the system bus BUS. The temperature sensor  42  and the humidity sensor  44  are also connected to the system bus BUS. Accordingly, CPU  60  obtains the combined resistance value measured by the resistance value measuring unit  76 , the temperatures measured by the temperature sensor  42  and the heating roller temperature measuring unit  78 , and the humidity measured by the humidity sensor  44 . 
     Action of the image forming apparatus  10  of the exemplary embodiment will be described below. First, a process flow of the image forming engine unit  74  will briefly be described. 
     The charger  14  charges the outer circumferential surface of the photoreceptor drum  12 , and the photoreceptor drum  12  and the intermediate transfer belt  20  are rotated. Then the laser beam scanning device  16  forms the electrostatic latent image on the photoreceptor drum  12 . The development device  18  supplies the toner to the electrostatic latent image, thereby developing the electrostatic latent image to obtain the toner image. The photoreceptor drum  12  conveys the toner image to a contact position (primary transfer position) with the intermediate transfer belt  20 . 
     The transfer power supply  28  supplies the electric power to the primary transfer roller  26 , and the primary transfer roller  26  presses the intermediate transfer belt  20  against the outer circumferential surface of the photoreceptor drum  12 , thereby transferring the toner image on the photoreceptor drum  12  to the image forming surface of the intermediate transfer belt  20 . That is, the toner image is conveyed by the photoreceptor drum  12  rotated in the direction of arrow A of  FIG. 1 , and the toner image is transferred to the outer circumferential surface of the intermediate transfer belt  20 . The toner image conveyed in the direction of the arrow B by the intermediate transfer belt  20  is transferred to the sheet P in a contact position (secondary transfer position) with the secondary transfer roller  38 , and the fixing device  40  fixes the toner image onto the sheet P. 
     When the image forming apparatus  10  forms the images for a long time to leave the intermediate transfer belt  20  in a high-temperature and high-humidity environment for a long time, a discharge product, such as nitrogen oxide and ozone, adhering to an inner circumferential surface (back side) of the intermediate transfer belt  20  absorbs moisture in air to deliquesces, and an electric resistance value on the inner circumferential surface of the intermediate transfer belt  20  is temporarily decreased. In such cases, a current passing through the primary transfer roller  26  temporarily becomes excessive, and there is a risk of a transfer defect being generated. When the inside of the image forming apparatus  10  is dried by heat generated from the member such as the heating roller  40 A having a heat source, the electric resistance value at the inner circumferential surface of the intermediate transfer belt  20  is returned to the original state. However, the transfer defect is inevitable until the electric resistance value is returned to the original state. 
     Therefore, in the image forming apparatus  10  of the exemplary embodiment, a transfer defect suppressing process is performed to suppress the transfer defect caused by the temporal decrease of the electric resistance value at the inner circumferential surface of the intermediate transfer belt  20  when the toner image is transferred from the photoreceptor drum  12  to the intermediate transfer belt  20 . 
     The operation of the image forming apparatus  10  in performing the transfer defect suppressing process during the image formation will be described with reference to  FIG. 3 .  FIG. 3  is a flowchart illustrating a flow of a transfer defect suppressing process program that is executed at predetermined time intervals (for example, every  0 . 5  second) by CPU  60  of the image forming apparatus  10  when the image forming engine unit  74  performs the image forming process. The transfer defect suppressing process program is previously stored in a predetermined region of ROM  62 . 
     Referring to  FIG. 3 , in Step  100 , assuming that the discharge product adhering to the inner circumferential surface of the intermediate transfer belt  20  deliquesces, it is determined whether or not predetermined conditions are satisfied. When the predetermined conditions are satisfied, the flow goes to Step  102 . When the predetermined conditions are not satisfied, the transfer defect suppressing process program is ended. 
     In the image forming apparatus  10  of the exemplary embodiment, assuming that the discharge product adhering to the inner circumferential surface of the intermediate transfer belt  20  deliquesces, the predetermined conditions are obtained from an experiment in which the real machine of the image forming apparatus  10  is used or a computer simulation based on design specifications of the image forming apparatus  10 . Specifically, the predetermined conditions are as follows: the temperature of the heating roller  40 A is equal to or lower than a predetermined temperature (for example, 40° C.), the temperature measured by the sensor  42  is equal to or more than a predetermined temperature (for example, 28° C.), the humidity measured by the humidity sensor  44  is equal to or more than predetermined humidity (for example, 70%), and an accumulative transfer amount of toner image transferred from the photoreceptor drum  12  to the intermediate transfer belt  20  reaches a predetermined amount (for example, 500000 images). However, any condition may be applied as long as the discharge product adhering to the inner circumferential surface of the intermediate transfer belt  20  deliquesces. 
     In Step  102 , the resistance value measuring unit  76  measures the combined resistance value of the photoreceptor drum  12 , intermediate transfer belt  20 , and primary transfer roller  26 . In the exemplary embodiment, a predetermined voltage is applied among the photoreceptor drum  12 , intermediate transfer belt  20 , and primary transfer roller  26 , a current passing through the photoreceptor drum  12 , intermediate transfer belt  20 , and primary transfer roller  26  is measured, and a resistance value computed based on the measured current and the applied voltage is set to the combined resistance value of the photoreceptor drum  12 , intermediate transfer belt  20 , and primary transfer roller  26 . Alternatively, the resistance values of the photoreceptor drum  12 , intermediate transfer belt  20 , and primary transfer roller  26  are separately measured and the combined resistance value may be computed based on the resistance values thereof. 
     In Step  104 , it is determined whether or not the combined resistance value measured by the resistance value measuring unit  76  in Step  102  is lower than a predetermined combined resistance value. When the combined resistance value is lower than the predetermined combined resistance value, the flow goes to Step  106 . When the combined resistance value is not lower than the predetermined combined resistance value, the transfer defect suppressing process program is ended. 
     In the exemplary embodiment, a value previously obtained from the experiment in which the real machine of the image forming apparatus  10  is used or the computer simulation based on design specifications of the image forming apparatus  10  is used as the predetermined combined resistance value in which the transfer defect is not generated when the toner image is transferred from the photoreceptor drum  12  to the intermediate transfer belt  20 . 
     In Step  106 , the control of the transfer power supply  28  is started such that the electric power supplied to the primary transfer roller  26  is gradually increased from an electric power smaller than a predetermined electric power (first supply value) to the predetermined electric power (second supply value) (hereinafter the control is referred to as “first electric power control”). 
     In the exemplary embodiment, assuming that the toner imaged is well transferred from the photoreceptor drum  12  to the intermediate transfer belt  20  while the decrease of the electric resistance value caused by the deliquescence of the discharge product adhering to the inner circumferential surface of the intermediate transfer belt  20  is not generated in the inner circumferential surface of the intermediate transfer belt  20 , an electric power previously obtained from the experiment in which the real machine of the image forming apparatus  10  is used or the computer simulation based on design specifications of the image forming apparatus  10  is used as the predetermined electric power. 
     In Step  108 , the combined resistance value of the photoreceptor drum  12 , intermediate transfer belt  20 , and primary transfer roller  26  is estimated. In the exemplary embodiment, the combined resistance value of the photoreceptor drum  12 , intermediate transfer belt  20 , and primary transfer roller  26  is estimated based on the accumulative transfer amount of toner image transferred from the photoreceptor drum  12  to the intermediate transfer belt  20 . That is, information indicating a correspondence relationship between the accumulative transfer amount of toner image transferred from the photoreceptor drum  12  to the intermediate transfer belt  20  and the combined resistance value of the photoreceptor drum  12 , intermediate transfer belt  20 , and primary transfer roller  26  is previously stored in the storage device such as ROM  62  or NVM  66 , and the information corresponding to the accumulative transfer amount of toner image transferred from the photoreceptor drum  12  to the intermediate transfer belt  20  is read from the storage device to obtain the combined resistance value indicated by the information. In another estimation method, information indicating a correspondence relationship between a physical quantity (for example, an operating time of the intermediate transfer belt  20 ) indicating the accumulative transfer amount of toner image transferred from the photoreceptor drum  12  to the intermediate transfer belt  20  and the combined resistance value of the photoreceptor drum  12 , intermediate transfer belt  20 , and primary transfer roller  26  is previously stored in the storage device such as ROM  62  or NVM  66 , and the information corresponding to the physical quantity indicating the accumulative transfer amount of toner image transferred from the photoreceptor drum  12  to the intermediate transfer belt  20  is read from the storage device to obtain the combined resistance value indicated by the information. 
     In Step  109 , the transfer power supply  28  is controlled such that the electric power corresponding to the combined resistance value estimated in Step  108  is supplied to the primary transfer roller  26 . In Step  109 , the electric power corresponding to the combined resistance value estimated in Step  108  is supplied to the primary transfer roller  26 . Alternatively, the electric power may be supplied to the primary transfer roller  26  according to the physical quantity (for example, the accumulative transfer amount of toner image or the operating time of the intermediate transfer belt  20 ) corresponding to the combined resistance value. 
     In Step  110 , it is determined whether or not the combined resistance value estimated in Step  108  reaches the predetermined combined resistance value. When the combined resistance value estimated in Step  108  reaches the predetermined combined resistance value, the flow goes to Step  120 . When the combined resistance value estimated in Step  108  does not reach the predetermined combined resistance value, the flow goes to Step  112 . 
     In Step  112 , the resistance value measuring unit  76  measures the combined resistance value of the photoreceptor drum  12 , intermediate transfer belt  20 , and primary transfer roller  26 . 
     In Step  114 , it is determined whether or not the combined resistance value measured by the resistance value measuring unit  76  in Step  112  is equal to or more than the predetermined combined resistance value. When the combined resistance value measured by the resistance value measuring unit  76  is equal to or more than the predetermined combined resistance value, the flow goes to Step  116 . When the combined resistance value measured by the resistance value measuring unit  76  is lower than the predetermined combined resistance value, the flow returns to Step  108 . 
     In Step  116 , the control of the transfer power supply  28  is started such that the electric power supplied to the primary transfer roller  26  reaches the predetermined electric power earlier than the electric power supplied from the transfer power supply  28  at the present time, and such that the electric power supplied to the primary transfer roller  26  is gradually increased to the predetermined electric power (hereinafter the control is referred to as “second electric power control”). 
       FIG. 4  is a graph illustrating an example of a relationship of an accumulative transfer amount of toner image transferred from the photoreceptor drum  12  to the intermediate transfer belt  20  from the beginning of a first electric power control, to an output level of the transfer power supply  28  by the first electric power control, or to an output level of the transfer power supply  28  by the second electric power control. In  FIG. 4 , a horizontal axis expresses the accumulative transfer amount of toner image (indicated by “Print Volume” in  FIG. 4 ), and a vertical axis expresses an output level (indicated by “Primary Transfer Output” in  FIG. 4 ). 
     As illustrated in  FIG. 4 , when the transfer power supply  28  has the output level of 100%, the electric power supplied to the primary transfer roller  26  becomes the predetermined electric power. In both the first electric power control and the second electric power control, the output level of the transfer power supply  28  is gradually increased in a stepwise manner from 50% to 100%. In the second electric power control, the output level of the transfer power supply  28  is gradually increased in the stepwise manner at time intervals shorter than that of the first electric power control so as to reach the predetermined electric power earlier than the first electric power control. 
       FIG. 4  also illustrates an example of the case in which a transition is made from the first electric power control to the second electric power control when the accumulative transfer amount of toner exceeds “100”. This is because the combined resistance value measured by the resistance value measuring unit  76  is equal to or more than the predetermined combined resistance value when the accumulative transfer amount of toner exceeds “100”. For example, when the combined resistance value measured by the resistance value measuring unit  76  is equal to or more than the predetermined combined resistance value at the time the accumulative transfer amount of toner reaches “200” or “300”, the transition is made at that time from the first electric power control to the second electric power control. 
     In Step  118 , the process waits until the condition that the electric power supplied to the primary transfer roller  26  reaches the predetermined electric power is satisfied, and thereafter the flow goes to Step  120 . 
     The condition that the electric power supplied to the primary transfer roller  26  reaches the predetermined electric power is used in Step  118 . Alternatively, for example, the condition that the accumulative transfer amount of toner image transferred from the photoreceptor drum  12  to the intermediate transfer belt  20  reaches the predetermined accumulative transfer amount or the condition that the physical quantity (for example, the operating time of the intermediate transfer belt  20 ) indicating the accumulative transfer amount of toner image transferred from the photoreceptor drum  12  to the intermediate transfer belt  20  reaches a predetermined threshold may be used. 
     In Step  120 , the control is performed such that the first electric power control is ended when the first electric power control is performed, and the control is performed such that the second electric power control is ended when the second electric power control is performed. Then the transfer defect suppressing process program is ended. 
     Although the exemplary embodiment of the invention is described above, the technical scope of the invention is not limited to the exemplary embodiment. Various changes and modifications can be made without departing from the gist of the invention, and the modes with such changes and modifications are included in the technical scope of the invention. 
     The invention is not limited to the exemplary embodiment, and all the combinations of features described in the exemplary embodiment are not necessary for the solving means of the invention. The exemplary embodiment includes the inventions of various stages, and various inventions can be extracted by a combination of the plural disclosed constituents depending on the situation. Even if some constituents are neglected from all the constituents described in the exemplary embodiment, the configuration in which some constituents are neglected can be extracted as the invention as long as the effect of the invention is obtained. 
     For example, in the exemplary embodiment, the combined resistance value of the photoreceptor drum  12 , intermediate transfer belt  20 , and primary transfer roller  26  is estimated based on the accumulative transfer amount of toner image transferred from the photoreceptor drum  12  to the intermediate transfer belt  20 . Alternatively, the combined resistance value of the photoreceptor drum  12 , intermediate transfer belt  20 , and primary transfer roller  26  may be estimated based on at least one of the temperature measured by the temperature sensor  42  and the humidity measured by the humidity sensor  44 . 
     As to the estimation method in this case, information indicating a correspondence relationship between at least one of the temperature and humidity in the space of the image forming apparatus  10  and the combined resistance value of the photoreceptor drum  12 , intermediate transfer belt  20 , and primary transfer roller  26  is previously stored in ROM  62  or NVM  66 , and the combined resistance value is estimated using the information. 
     In the exemplary embodiment, the combined resistance value is estimated and the first electric power control is ended when the estimated combined resistance value becomes the predetermined combined resistance value. Alternatively, the first electric power control may be ended when the combination of the temperature measured by the temperature sensor  42  and the humidity measured by the humidity sensor  44  becomes a predetermined combination. 
     Assuming that the combined resistance value of the photoreceptor drum  12 , intermediate transfer belt  20 , and primary transfer roller  26  becomes the predetermined combined resistance value, a combination of the temperature and the humidity previously obtained from the experiment in which the real machine of the image forming apparatus  10  is used or the computer simulation based on design specifications of the image forming apparatus  10  is used as the predetermined combination. 
     In the exemplary embodiment, the electric power supplied to the primary transfer roller  26  is gradually increased in a stepwise manner to a predetermined electric power from an electric power smaller than the predetermined electric power. Alternatively, the electric power supplied to the primary transfer roller  26  may continuously gradually be increased to a predetermined electric power from an electric power smaller than the predetermined electric power. 
     In the exemplary embodiment, transfer power supply  28  is controlled such that the electric power supplied to the primary transfer roller  26  is gradually increased to a predetermined electric power from an electric power smaller than the predetermined electric power. The electric power supplied to the secondary transfer roller  38  may similarly be controlled. 
     In the exemplary embodiment, it is determined whether or not the estimated combined resistance value reaches the predetermined combined resistance value. Alternatively, it may be determined whether or not the physical quantity (in this case, the accumulative transfer amount of toner image transferred from the photoreceptor drum  12  to the intermediate transfer belt  20 ) corresponding to the combined resistance value reaches a predetermined physical quantity (in this case, a predetermined accumulative transfer amount of toner image). Assuming that the combined resistance value of the photoreceptor drum  12 , intermediate transfer belt  20 , and primary transfer roller  26  becomes the predetermined combined resistance value, a value previously obtained from the experiment in which the real machine of the image forming apparatus  10  is used or the computer simulation based on design specifications of the image forming apparatus  10  is used as the predetermined accumulative transfer amount of toner image. 
     In the exemplary embodiment, the rotary-type development device  18  is used to superimpose the Y, M, C, and K toner images. Alternatively, the development section that forms the Y toner image, the development section that forms the M toner image, the development section that forms the C toner image, and the development section that forms the K toner image may be arranged along the outer circumferential surface of the photoreceptor drum  12 . Y, M, C, and K image forming units each of which includes the development section, the photoreceptor drum  12 , the charger  14 , the laser beam scanning device  16 , and the erasing and cleaning device  22  may be arranged in parallel on the intermediate transfer belt  20 . 
     The configuration (see  FIG. 1 ) of the image forming apparatus  10  of the exemplary embodiment is described only by way of example. The configuration of the image forming apparatus  10  of the exemplary embodiment may be changed without departing from the scope of the invention depending on the situation. 
     The process flow (see  FIG. 3 ) of the transfer defect suppressing process program described in the exemplary embodiment is also only by way of example. The unnecessary step may be neglected, a new step may be added, or a process sequence may be changed without departing from the scope of the invention. 
     The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The exemplary embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.