Patent Publication Number: US-8995853-B2

Title: Transfer device and image forming apparatus including transfer device

Description:
The application is based on Japanese Patent Application No. 2012-261967 filed at the Japanese Patent Office on Nov. 30, 2012, and the contents thereof are incorporated herein by reference. 
     BACKGROUND 
     The disclosure relates to a transfer device with an intermediate transfer belt that carries a toner image and an image forming apparatus including the transfer device. 
     An electrophotographic image forming apparatus is known which includes a photosensitive drum that carries an electrostatic latent image and a transfer device that transfers a toner image from the photosensitive drum to a sheet. To transfer an image in a plurality of colors to the sheet, the transfer device includes an intermediate transfer belt, a primary transfer member, and a secondary transfer member. The intermediate transfer belt rotates circumferentially opposite a plurality of photosensitive drums. A primary transfer voltage applied to the primary transfer member allows the toner image to be transferred from each of the photosensitive drums onto the intermediate transfer belt. Then, a secondary transfer voltage applied to the secondary transfer member allows the toner images to be transferred from the intermediate transfer belt to the sheet at a time. Furthermore, a technique is known in which, to inhibit possible color misalignment in a high or low humidity environment, the rotation speeds of the photosensitive members for the respective colors are individually adjusted to rank the speed ratios between the intermediate transfer belt and the photosensitive members. 
     SUMMARY 
     A transfer device according to an aspect of the disclosure includes an intermediate transfer belt, a driving roller, an environment sensor, a resistance detecting sensor, and a speed ratio adjusting section. The intermediate transfer belt is disposed to face a plurality of image carriers each carrying a toner image on a circumferential surface of the image carrier and moreover rotationally driven at an equal speed. The intermediate transfer belt is circumferentially and rotationally driven in one direction. The toner images from the plurality of image carriers are transferred onto a surface of the intermediate transfer belt in a superimposed manner. The intermediate transfer belt is supported around the driving roller, and the driving roller circumferentially and rotationally drives the intermediate transfer belt. The environment sensor detects a temperature or a humidity around the intermediate transfer belt. The resistance detecting section detects a change in a resistance value of the intermediate transfer belt. The speed ratio adjusting section controls a rotation speed of the driving roller according to results of detection by the environment sensor and the resistance detecting section, thereby adjusting a speed ratio Vb/Vd between a circumferential rotating speed Vb of the intermediate transfer belt and a rotation speed Vd of the image carrier. 
     Furthermore, an image forming apparatus according to another aspect of the disclosure includes the above-described transfer device, the above-described plurality of image carriers, and a secondary transfer roller. The secondary transfer roller transfers the toner images from the intermediate transfer belt to a sheet. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a cross-sectional view showing the internal structure of an image forming apparatus according to an embodiment of the disclosure; 
         FIG. 2  is a schematic cross-sectional view showing a peripheral portion of an intermediate transfer unit according to the embodiment of the disclosure; 
         FIG. 3  is an electrical block diagram of a control section according to the embodiment of the disclosure; 
         FIG. 4  is a graph showing a relation between color misalignment and the speed ratio between an intermediate transfer belt and a photosensitive drum; 
         FIG. 5  is a graph showing the relation between color misalignment and the speed ratio between the intermediate transfer belt and the photosensitive drum; and 
         FIG. 6  is a flowchart showing a control aspect of driving control according to the embodiment of the disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     An image forming apparatus  10  according to an embodiment of the disclosure will be described below in detail with reference to the drawings. The present embodiment illustrates a tandem color printer as an example of an image forming apparatus. The image forming apparatus may be, for example, a copier, a facsimile machine, and a multifunction machine including a copier and a facsimile machine. 
       FIG. 1  is a cross-sectional view showing the internal structure of the image forming apparatus  10 . Furthermore,  FIG. 2  is a schematic cross-sectional view showing a peripheral portion of an intermediate transfer unit  14  inside the image forming apparatus  10 . The image forming apparatus  10  includes an apparatus main body  11  with a box-shaped housing structure. The apparatus main body  11  contains a sheet feeding section  12  that feeds a sheet P, an image forming section  13  that forms a toner image to be transferred onto the sheet P fed from the sheet feeding section  12 , an intermediate transfer unit  14  (transfer device) to which the toner image is primarily transferred, a secondary transfer roller  145 , a toner supply section  15  that supplies toner in the image forming section  13 , and a fixing section  16  that carries out a process of fixing an unfixed image formed on the sheet P, to the sheet P. Moreover, a sheet discharge section  17  into which that the sheet P on which a fixing process has been carried out by the fixing section  16  is discharged is provided above the apparatus main body  11 . 
     An operation panel (not shown in the drawings) is provided on an upper surface of the apparatus main body  11  at an appropriate position so that an operation of inputting, for example, output conditions for the sheet P is performed via the operation panel. The operation panel includes a power supply key, and a touch panel and various operations keys via which the output conditions are input. 
     Moreover, in the apparatus main body  11 , a sheet conveying path  111  extending in an up-down direction is formed to at a right-hand position with respect to the image forming section  13 . A conveying roller pair  112  is provided at an appropriate position on the sheet conveying path  111  to convey the sheet to an appropriate position. Furthermore, a registration roller pair  113  is provided on the sheet conveying path  111  upstream of a secondary transfer nip portion, which is described later, to carry out skew correction on the sheet and to feed the sheet into the nip portion at a predetermined timing. The sheet conveying path  111  is a conveying path along which the sheet P is conveyed from the sheet feeding section  12  to the sheet discharge section  17  via the image forming section  13  and the fixing section  16 . 
     The sheet feeding section  12  includes a sheet feeding tray  121 , a pickup roller  122 , and a sheet feeding roller pair  123 . The sheet feeding tray  121  is removably installed at a lower position in the apparatus main body  11  so that a bundle of sheets P1 with a plurality of sheets P stacked therein can be stored in the sheet feeding tray  121 . The pickup roller  122  sends out the uppermost sheet P of the bundle of sheets P1 stored in the sheet feeding tray  121  one by one. The sheet feeding roller pair  123  feeds the sheet P sent out by the pickup roller  122  out to the sheet conveying path  111 . 
     The sheet feeding section  12  includes a manual sheet feeding section attached to left-hand side surface of the apparatus main body  11  shown in  FIG. 1 . The manual sheet feeding section includes a manual feed tray  124 , a pickup roller  125 , and a sheet feeding roller pair  126 . The manual feed tray  124  is a tray on which the sheet P to be manually fed is placed and which is opened from a side surface of the apparatus main body  11  as shown in  FIG. 1  when the sheet P is manually fed. The pickup roller  125  sends out the sheet P placed on the manual feed tray  124 . The sheet feeding roller pair  126  feeds the sheet P sent out by the pickup roller  125  out to the sheet conveying path  111 . 
     The image forming section  13  forms a toner image to be transferred to the sheet P, and includes a plurality of image forming units forming toner images in different colors. As the image forming units, the present embodiment includes a magenta unit  13 M using a magenta (M) developer, a cyan unit  13 C using a cyan (C) developer, a yellow unit  13 Y using a yellow (Y) developer, and a black unit  13 Bk using a black (Bk) developer; the magenta unit  13 M, the cyan unit  13 C, the yellow unit  13 Y, and the black unit  13 Bk are sequentially disposed from an upstream side toward a downstream side (from a left-hand side to a right-hand side shown in  FIG. 1 ) in a rotating direction (circumferential rotating direction) of an intermediate transfer belt  141  described below. Each of the units  13 M,  13 C,  13 Y, and  13 Bk includes a photosensitive drum  20  (image carrier), and a charging device  21 , a developing device  23 , and a cleaning device  25  disposed around the photosensitive drum  20 . Furthermore, an exposure device  22  common to the units  13 M,  13 C,  13 Y, and  13 Bk is disposed below the image forming units. 
     The photosensitive drum  20  is rotationally driven around a shaft thereof. An electrostatic latent image and a toner image are formed on a peripheral surface of the photosensitive drum  20 . The photosensitive drum  20  may be a photosensitive drum formed of a material containing amorphous silicon (a-Si). As shown in  FIG. 2 , photosensitive drums  20 M,  20 C,  20 Y, and  20 Bk are disposed in association with the image forming units for the respective colors. The charging device  21  uniformly charges a surface of the photosensitive drum  20 . The charging device  21  is a charging device based on a contact charging scheme and including a charging roller and a charge cleaning brush for removing toner attached to the charging roller. The exposure device  22  has various types of optical equipment such as a light source, a polygon mirror, a reflection mirror, and a deflection mirror. The exposure device  22  irradiates the uniformly charged peripheral surface of the photosensitive drum  20  with light modulated based on image data to form an electrostatic latent image. The photosensitive drum  20  is rotationally driven by a drum motor  61  ( FIG. 2 ). According to the present embodiment, the photosensitive drums  20  corresponding to the image forming units for the respective colors are rotationally driven by the drum motor  61  at an equal speed. Additionally, the cleaning device  25  cleans the peripheral surface of the photosensitive drum  20  after transfer of a toner image. 
     In order to develop an electrostatic latent image formed on the photosensitive drum  20 , the developing device  23  supplies toner to the peripheral surface of the photosensitive drum  20 . The developing device  23  is for two-component developer containing toner and a carrier, and includes two agitating rollers  23 A, a magnetic roller  23 B, and a developing roller  23 C. The agitating rollers  23 A cyclically convey and agitate the two-component developer to charge the toner. A two-component developer layer is carried on a peripheral surface of the magnetic roller  23 B. A toner layer is carried on a peripheral surface of the developing toner  23 C; the toner layer is formed when the toner is delivered from the magnetic roller  23 B to the developing toner  23 C due to a potential difference between the magnetic roller  23 B and the developing roller  23 C. The toner on the developing roller  23 C is supplied to the peripheral surface of the photosensitive drum  20 , and the electrostatic latent image is developed. According to the present embodiment, the toner is characterized by being charged to a positive polarity. 
     The intermediate transfer unit  14  is disposed in a space provided between the image forming section  13  and the toner supply section  15 . As shown in  FIG. 2 , the intermediate transfer unit  14  includes an intermediate transfer belt  141 , a driving roller  142 , a driven roller  143 , and a primary transfer roller  24 . 
     The intermediate transfer belt  141  is an endless belt-like rotator and is stretched and supported around the driving roller  142  and the driven roller  143  so that a peripheral surface side of the intermediate transfer belt  141  comes into abutting contact with the peripheral surface of each photosensitive drum  20 . The intermediate transfer belt  141  is a conductive soft belt with a stack structure including a base layer, an elastic layer, and a coat layer. The base layer forms the lowermost layer of the intermediate transfer belt  141 . The base layer is preferably, for example, polyvinylidene difluoride (PVDF) or a polyimide resin. The elastic layer applies appropriate elasticity to the intermediate transfer belt  141 . A material for the elastic layer is, for example, hydrin rubber, chloroprene rubber, or polyurethane rubber. Furthermore, the coat layer forms the uppermost layer of the intermediate transfer belt  141  and comes into contact with the photosensitive drum  20  ( FIG. 1 ). The coat layer protects the elastic layer, and a material for the coat layer is acryl, silicon, or a fluorine resin such as PTFE. 
     The driving roller  142  is located at a right end side of the intermediate transfer unit  14 . The intermediate transfer belt  141  is stretched and supported around and driven by the driving roller  142  for circumferential rotation. The driving roller  142  is provided with a rotational driving force by a belt motor  60  described below. The driving roller  142  is formed of a metal roller. 
     More specifically, the driving roller  142  has a two-layer structure in a cross-sectional direction. An inner layer of the driving roller  142  is formed of an aluminum layer. Furthermore, an alumite layer forming a surface layer of the driving roller  142  is provided on the aluminum layer. According to the present embodiment, the alumite layer is an alumite coated film corresponding to the anodized aluminum layer. When the intermediate transfer belt  141  is internally supported with the alumite layer in contact with an inner peripheral surface of the intermediate transfer belt  141 , a rotational driving force is suitably transmitted from the driving roller  142  to the intermediate transfer belt  141 . 
     A well-known film formation method may be used to form an alumite film forming the alumite layer. According to the present embodiment, an element pipe formed of an aluminum pipe is used, and an alumite film is formed on a surface of the element pipe by an anodization method using dilute sulfuric acid as an electrolyte. The film formation results in a porous film formed on the surface of the alumite film and including a barrier layer and a countless number of fine pores. If the fine pores remain open, foreign matter and humic substances may be absorbed by the film. Thus, preferably a sealing process is carried out as aftertreatment. This allows the fine pores in the surface of the film to be occluded, improving corrosion resistance, weather resistance, and contamination resistance. 
     The driven roller  143  is located at a left end side of the intermediate transfer unit  14 , and the intermediate transfer belt  141  is stretched and supported around the driven roller  143 . The driven roller  143  applies tension to the intermediate transfer belt  141 . A belt cleaning device  144  ( FIG. 1 ) is disposed near the driven roller  143  to remove toner remaining on the peripheral surface of the intermediate transfer belt  141 . 
     The primary transfer roller  24  forms the primary transfer nip portion with the photosensitive drum  20  across the intermediate transfer belt  141 . The primary transfer roller  24  primarily transfers a toner image on the photosensitive drum  20  onto the intermediate transfer belt  141 . As shown in  FIG. 2 , primary transfer rollers  24 M,  24 C,  24 Y, and  24 Bk are disposed opposite the photosensitive drums  20  for the respective colors. The primary transfer nip portion is formed between each photosensitive drum  20  and the corresponding primary transfer roller  24 . 
     A secondary transfer roller  145  is disposed opposite the driving roller  142 . The secondary transfer roller  145  is compressed against the peripheral surface of the intermediate transfer belt  141  to form a secondary transfer nip portion. The toner image primarily transferred onto the intermediate transfer belt  141  is secondarily transferred, at the secondary transfer nip portion, to the sheet P fed from the sheet feeding section  12 . According to the present embodiment, the secondary transfer roller  145  is formed of epichlorohydrin. 
     The toner supply section  15  stores toner used for image formation, and according to the present embodiment, includes a magenta toner container  15 M, a cyan toner container  15 C, a yellow toner container  15 Y, and a black toner container  15 Bk. The toner containers  15 M,  15 C,  15 Y, and  15 Bk store refill toner in the respective colors M, C, Y, and Bk, and supply toner in the respective colors to the developing devices  23  of the corresponding image forming units  13 M,  13 C,  13 Y, and  13 Bk for the respective colors M, C, Y, and Bk, through toner discharge ports  15 H formed in bottom surfaces of the corresponding containers and through toner conveying sections (not shown in the drawings). 
     The fixing section  16  includes a heating roller  161  with a heating source provided inside, a fixing roller  162  disposed opposite the heating roller  161 , a fixing belt  163  stretched and supported around the fixing roller  162  and the heating roller  161 , and a pressing roller  164  disposed opposite the fixing roller  162  across the fixing belt  163  to form a fixing nip portion. The sheet P fed to the fixing section  16  passes through the fixing nip portion, where the sheet P is heated and pressurized. Thus, the toner image transferred to the sheet P at the secondary transfer nip portion is fixed to the sheet P. 
     The sheet discharge section  17  is a recessed portion of a top portion of the apparatus main body  11  including a sheet discharge tray  171  formed at a bottom portion of the recess portion and in which the discharged sheet P is received. The sheet P on which a fixing process has been carried out passes along the sheet conveying path  111  extending from the upper portion of the fixing section  16  and is then discharged toward the sheet discharge tray  171 . 
     As shown in  FIG. 2 , the intermediate transfer unit  14  further includes a first backup roller  146  (backup roller), a second backup roller  147 , a belt motor  60 , a density sensor  62 , an environment sensor  63 , and a control section  90  ( FIG. 3 ). 
     The first backup roller  146  is disposed between the driving roller  142  and the primary transfer roller  24 Bk, positioned on the most downstream side among the plurality of primary transfer rollers  24  in the circumferential rotating direction of the intermediate transfer belt  141 . The intermediate transfer belt  141  is stretched and supported around the first backup roller  146 . More specifically, the intermediate transfer belt  141  is stretched around a peripheral surface of the first backup roller  146  at a predetermined angle. Likewise, the second backup roller  147  is disposed between the driven roller  143  and the primary transfer roller  24 M, positioned on the most upstream side among the plurality of primary transfer rollers  24  in the circumferential rotating direction of the intermediate transfer belt  141 . The intermediate transfer belt  141  is stretched and supported around the second backup roller  147 . The first backup roller  146  and the second backup roller  147  support the intermediate transfer belt  141  to linearly arrange the plurality of primary transfer nip portions. When the image forming apparatus  10  is not in operation, the primary transfer rollers  24  for the respective colors and the second backup roller  147  can move toward the inside of the intermediate transfer belt  141 . That is, the primary transfer roller  24  can be separated from the photosensitive drum  20 . At this time, the intermediate transfer belt  141  is stretched and supported between the first backup roller  146  and the driven roller  143 . When the primary transfer roller  24  is separate from the photosensitive drum  20 , the intermediate transfer belt  141  or the photosensitive drum  20  can be installed in and removed from the apparatus main body  11  without bringing the intermediate transfer belt  141  and the photosensitive drum  20  into contact with each other. 
     The belt motor  60  generates a driving force for circumferentially rotating the intermediate transfer belt  141 . The belt motor  60  transmits the rotational driving force to the driving roller  142 . The belt motor  60  is controlled by a driving control section  91  described below to change the rotation speed of the driving roller  142 . 
     The density sensor  62  (resistance detector) is disposed between the photosensitive drum  20 Bk for black and the first backup roller  146  opposite a belt surface of the intermediate transfer belt  141 . The density sensor  62  detects the density of the toner image formed on the intermediate transfer belt  141 . Moreover, the density sensor  62  detects the density of the belt surface of the intermediate transfer belt  141 . The density of the belt surface of the intermediate transfer belt  141  detected by the density sensor  62  is referenced by a speed adjusting section  92  to detect a change in the resistance value of the intermediate transfer belt  141 . 
     The environment sensor  63  is provided in the apparatus main body  11  of the image forming apparatus  10  to detect ambient temperature and humidity. Temperature and humidity data detected by the environment sensor  63  is stored in a storage section  93  described below. Furthermore, the temperature and humidity data is referenced by the speed adjusting section  92  to determine whether or not to perform speed adjusting control on the intermediate transfer belt  141 . 
     The control section  90  is configured by a central processing unit (CPU), read only memory (ROM) that stores control programs, random access memory (RAM) used as work areas for the CPU, or the like.  FIG. 3  is an electrical block diagram of the control section  90 . The control section  90  connects electrically to the belt motor  60 , the drum motor  61 , the density sensor  62 , and the environment sensor  63 . The CPU executes the control programs stored in the ROM to allow the control section  90  to function as the driving control section  91 , the speed adjusting section  92  (speed ratio adjusting section), and a storage section  93 , which are provided in the control section  90 . 
     The driving control section  91  controls the drum motor  61  to change the rotation speed Vd of the photosensitive drum  20 . The driving control section  91  also controls the belt motor  60  to change the circumferential rotating speed Vb of the intermediate transfer belt  141 . 
     The speed adjusting section  92  controls the rotation speed of the driving roller  142  according to the results of detection by the environment sensor  63  and the density sensor  62  to adjust a speed ratio Vb/Vd between the circumferential rotating speed Vb of the intermediate transfer belt  141  and the rotation speed Vd of the photosensitive drum  20 . 
     The storage section  93  stores a temperature threshold T0. The temperature threshold T0 is compared with a detected temperature T detected by the environment sensor  63  by the speed adjusting section  92 . The storage section  93  also stores a density threshold D0 for the belt surface. The speed adjusting section  92  references and compares the density threshold D0 with a detected density D for the belt surface detected by the density sensor  62 . According to another embodiment, the temperature threshold T0 may be a threshold for humidity or for a combination of temperature and humidity. 
     &lt;Occurrence and Elimination of Color Misalignment&gt; 
     Now, color misalignment that may occur on the intermediate transfer belt  141  will be described.  FIG. 4  and  FIG. 5  are graphs showing the relation between color misalignment and the speed ratio between the intermediate transfer belt  141  and the photosensitive drum  20 .  FIG. 4  is a diagram showing the occurrence level of color misalignment in different temperature and humidity environments in an initial stage of the use of the image forming apparatus  10 . Furthermore,  FIG. 5  is a graph showing the occurrence level of color misalignment before and after the surface of the intermediate transfer belt  141  is whitened as a result of the use of the image forming apparatus  10  for a predetermined period. The color misalignment means misalignment of toner images that may occur on the intermediate transfer belt  141  when the toner images are transferred from the photosensitive drum  20  to the intermediate transfer belt  141 . 
     As described above, in the intermediate transfer unit  14  according to the present embodiment, the rotation speed of the driving roller  142  is changed to enable adjustment of the speed ratio Vb/Vd between the circumferential rotating speed Vb of the intermediate transfer belt  141  and the rotation speed Vd of the photosensitive drum  20 . As shown in  FIG. 4 , in the initial stage of the use of the image forming apparatus  10 , in other words, when no external additive for toner or the like is stuck to the surface of the intermediate transfer belt  141 , possible color misalignment is suppressed in a normal temperature environment (A) (temperature: 23° C., humidity: 50%) in a region where the speed ratio Vb/Vd is lower than 1.0. More specifically, in a region where the speed ratio Vb/Vd is equal to or higher than 0.75 and equal to or lower than 0.85, the minimum value is present at which possible color misalignment is maximally suppressed. 
     On the other hand, in a high temperature environment (B) (temperature: 32.5° C., humidity: 80%), possible color misalignment is suppressed in a region where the speed ratio is Vb/Vd is higher than 1.0. More specifically, in a region where the speed ratio Vb/Vd is equal to or higher than 1.05 and equal to or lower than 1.15, the minimum value is present at which possible color misalignment is maximally suppressed. 
     As described above, in the present embodiment, a metal roller is used as the driving roller  142 . In particular, the driving roller  142  includes the layer formed of alumite. Such a metal roller has its diameter changed by temperature or humidity less significantly than elastic rollers. This enables suppression of a variation in the speed of the intermediate transfer belt  141  associated with the change in diameter. On the other hand, slippage may occur intermediate transfer belt  141  between the metal roller and the inner peripheral surface of the intermediate transfer belt  141 . Such slippage makes the circumferential rotational driving of the intermediate transfer belt  141  unstable, leading to an increased likelihood of the color misalignment. Moreover, the slippage between the driving roller  142  and the intermediate transfer belt  141  is likely to be significant in a high-temperature and high-humidity environment. Thus, in the high-temperature and high-humidity environment, the adverse effect of the slippage is pre-mitigated to reduce the likelihood of color misalignment in a region where the intermediate transfer belt  141  is circumferentially and rotationally driven faster than the photosensitive drum  20  as shown in  FIG. 4 . 
     Moreover, the after endurance (C) in  FIG. 5  means that the intermediate transfer belt  141  is used for a long period and is indicative of the occurrence level of color misalignment occurring when an external additive for toner adheres to the surface of the intermediate transfer belt  141 . The after endurance (C) data has been obtained through evaluation in the above-described normal temperature environment. The initial (A) data in  FIG. 5  corresponds to the data on the normal temperature environment (A) in  FIG. 4 . When the external additive for toner adheres to the surface of the intermediate transfer belt  141 , the resistance value of the intermediate transfer belt  141  increases. This increases an attractive force exerted between the intermediate transfer belt  141  and the photosensitive drum  20 . In this case, when the intermediate transfer belt  141  is circumferentially and rotationally driven faster than the photosensitive drum  20 , the occurrence of color misalignment is actually observed because the speed is likely to vary under the attractive force. As shown in  FIG. 5 , the characteristics observed after endurance (C) tend to be more similar to the characteristics observed in the normal temperature environment (A) than to the characteristics observed in the above-described high temperature environment (B). Specifically, after endurance (C), possible color misalignment is suppressed in a region where the speed ratio Vb/Vd is lower than 1.0. More specifically, the minimum value at which possible color misalignment is maximally suppressed is present in a region where the speed ratio Vb/Vd is equal to or higher than 0.85 and equal to or lower than 0.95. Thus, after endurance (C), when the attractive force between the intermediate transfer belt  141  and the photosensitive drum  20  is likely to increase, color misalignment caused by the attractive force is suppressed by allowing the intermediate transfer belt  141  and the photosensitive drum  20  to rotate at an equal speed. 
     As described above, according to the present embodiment, the primary transfer roller  24  includes a retract mechanism, and the first backup roller  146  is disposed between the primary transfer roller  24 Bk for black and the driving roller  142  ( FIG. 2 ). The first backup roller  146  forms a horizontal and linear primary transfer area between the first backup roller  146  and the second backup roller  147 . A secondary transfer area extending from a lower side to an upper side is formed between the driving roller  142  and the secondary transfer roller  145  by the first backup roller  146 . The discloser has found that, in the normal temperature environment (A) in  FIG. 4 , toner splashes significantly in the secondary transfer area when the speed ratio Vb/Vd is equal to or higher than 1.0 (area X in  FIG. 4 ). In a manufacture stage of the intermediate transfer belt  141 , a memory corresponding to a curved portion of the belt surface of the intermediate transfer belt  141  may remain on the belt surface. When the intermediate transfer belt  141  is circumferentially and rotationally driven faster than the photosensitive drum  20 , splash of toner is observed because the memory on the belt surface is emphasized between the driving roller  142  and the first backup roller  146  to change the behavior of the belt in the secondary transfer area. 
     &lt;Adjustment of the Speed Ratio Vb/Vd&gt; 
     To solve the above-described problems, the speed adjusting section  92  adjusts the speed ratio Vb/Vd according to the present embodiment. Speed control will be described which is performed by the speed adjusting section  92  in the intermediate transfer unit  14  according to the present embodiment.  FIG. 6  is a flowchart showing a control aspect of the speed control according to the present embodiment. 
     When the image forming apparatus  10  is turned on (step S 001 ), the speed adjusting section  92  determines whether or not to adjust the speed ratio Vb/Vd between the circumferential rotating speed Vb of the intermediate transfer belt  141  and the rotation speed Vd of the photosensitive drum  20 . The speed adjusting section  92  compares a detected temperature T detected by the environment sensor  63  with a temperature threshold T0 prestored in the storage section  93  (step S 002 ). According to the present embodiment, the temperature threshold T0 is set to 28° C. by way of example. If the detected temperature T is lower than the temperature threshold T0 (YES in step S 002 ), the speed adjusting section  92  determines that high temperature control on the speed ratio Vb/Vd is unnecessary. In this case, the speed adjusting section  92  sets the speed ratio Vb/Vd to Vr1 (first speed ratio). Based on the data on the normal temperature environment (A) in  FIG. 4  described above, Vr1 is set to lower than 1.0. More preferably, Vr1 is set equal to or higher than 0.75 and equal to or lower than 0.85. As a result, in the normal temperature environment, a toner image formed on the intermediate transfer belt  141  is suitably inhibited from being subjected to color misalignment. Moreover, even if the curved memory is present on the belt surface, the splash of toner in the secondary transfer area is inhibited by setting Vr1 to lower than 1.0. 
     On the other hand, if the detected temperature T is equal to or higher than the temperature threshold T0 (NO in step S 002 ), the speed adjusting section  92  determines that the high temperature control on the speed ratio Vb/Vd needs to be performed. In this case, the speed adjusting section  92  further checks the belt for whitening (step S 004 ). That is, the speed adjusting section  92  compares the detected density D of the belt surface detected by the density sensor  62  with a density threshold D0 stored in advance in the storage section  93 . The surface of the intermediate transfer belt  141  is normally black but is whitened when the external additive for toner is attached to the surface. The density threshold D0 is preset in association with the density of the whitened belt surface of the intermediate transfer belt  141 . If the result of the comparison of the detected density D indicates that the belt surface of the intermediate transfer belt  141  has not been whitened (NO in step S 004 ), the speed adjusting section  92  performs the high temperature control on the speed ratio Vb/Vd in association with the normal belt surface. In this case, the speed adjusting section  92  sets the speed ratio Vb/Vd to Vr2 (second speed ratio) (step S 005 ). Based on the data on the high temperature environment (B) in  FIG. 4  described above, Vr2 is set equal to or higher than 1.0. More preferably, Vr2 is set equal to or higher than 1.05 and equal to or lower than 1.15. As a result, in the high temperature environment, the toner image formed on the intermediate transfer belt  141  is suitably inhibited from being subjected to color misalignment. 
     Moreover, if the result of comparison of the detected density D indicates that the belt surface of the intermediate transfer belt  141  is whitened (YES in step S 004 ), the speed adjusting section  92  performs high resistance control on the speed ratio Vb/Vd in association with the whitened belt surface. In this case, the speed adjusting section  92  sets the speed ratio Vb/Vd to Vr3 (third speed ratio) in preference to the above-described Vr2 (step S 006 ). Based on the after endurance (C) data in  FIG. 5 , Vr3 is set to lower than 1.0. More preferably, Vr3 is set equal to or higher than 0.85 and equal to or lower than 0.95. As a result, even if the belt surface of the intermediate transfer belt  141  is whitened, the toner image formed on the intermediate transfer belt  141  is suitably inhibited from being subjected to color misalignment. 
     As described above, the intermediate transfer belt  141  is disposed opposite the plurality of photosensitive drums  20  rotationally driven at an equal speed. The rotation speed of the driving roller  142 , which circumferentially and rotationally drives the intermediate transfer belt  141 , is controlled to adjust the speed ratio Vb/Vd between the circumferential rotating speed Vb of the intermediate transfer belt  141  and the rotation speed Vd of the photosensitive drum  20 . Thus, the speed ratio Vb/Vd can be adjusted using a simpler configuration than in a case where the rotation speeds of the plurality of photosensitive drums  20  are individually adjusted. Furthermore, the speed adjusting section  92  adjusts the speed ratio according to the results of detection by the environment sensor  63  and the density sensor  62 . Thus, the speed ratio can be adjusted if the likelihood of slippage between the intermediate transfer belt  141  and the driving roller  142  changes in conjunction with a change in the environment or the attractive force exerted between the intermediate transfer belt  141  and the photosensitive drum  20  changes in conjunction with a change in the resistance value of the intermediate transfer belt  141 . This enables the toner image transferred from the photosensitive drum  20  to the intermediate transfer belt  141  to be inhibited from undergoing color misalignment as a result of a change in the likelihood of slippage or the attractive force. 
     In particular, when the temperature or humidity detected by the environment sensor  63  is equal to or higher than the predetermined threshold, the second speed ratio Vr2, which is higher than the first speed ratio Vr1, is set. Thus, in the high temperature or humidity environment, the toner image is suitably inhibited from being subjected to color misalignment even if the intermediate transfer belt  141  and the driving roller  142  are likely to slip with respect to each other. 
     Furthermore, when the density sensor  62  detects that the resistance value of the intermediate transfer belt  141  has increased above a preset threshold, a third speed ratio Vr3 is set which is higher than the first speed ratio Vr1 and lower than the second speed ratio Vr2. That is, when the resistance value of the intermediate transfer belt  141  rises to increase the attractive force exerted between the intermediate transfer belt  141  and the photosensitive drum  20 , the third speed ratio Vr3 is set, which is lower than the second speed ratio Vr2 for the high temperature or humidity environment. Thus, even when the circumferential rotating speed of the intermediate transfer belt  141  is likely to vary as a result of an increase in the attractive force, the speed ratio is inhibited from being set to an excessively large value. This inhibits color misalignment caused by a variation in the circumferential rotating speed. 
     Furthermore, the density sensor  62  functions as a resistance detecting section. Thus, the density sensor  62  suitably detects that the resistance value of the intermediate transfer belt  141  is changed by attachment of the external additive for toner or the like to the belt surface of the intermediate transfer belt  141 . In this case, the density sensor  62  serves both as a mechanism that detects a change in the resistance value of the intermediate transfer belt  141  and as a mechanism that detects the density of the toner image on the intermediate transfer belt  141 . 
     Additionally, the metal roller used as the driving roller  142  can be inhibited from having its diameter changed depending on temperature or humidity, compared to elastic rollers. This enables inhibition of a variation in the speed of the intermediate transfer belt  141  associated with the change in diameter. In addition, even when the metal roller and the intermediate transfer belt  141  are likely to slip with respect to each other, the speed ratio between the intermediate transfer belt  141  and the photosensitive drum  20  can be adjusted. Moreover, the driving roller  142 , including the alumite layer, and the intermediate transfer belt  141  are likely to electostatically attract each other. Hence, the rotational driving force of the driving roller  142  is stably transmitted to the intermediate transfer belt  141 . 
     Furthermore, in a relatively low temperature or humidity environment, the intermediate transfer belt  141  is inhibited from being circumferentially and rotationally driven faster than the photosensitive drum  20 . This restrains excessive tension from being applied to the belt surface between the first backup roller  146  and the driving roller  142 . Thus, even when, in the manufacture of the intermediate transfer belt  141 , a memory corresponding to a curved portion of the belt surface remains, the memory is prevented from appearing at the secondary transfer nip due to the tension. This inhibits possible splash of toner. 
     Additionally, in the image forming apparatus  10  including the intermediate transfer unit  14  according to the embodiment, the toner image transferred onto the sheet is inhibited from being subjected to color misalignment. 
     One embodiment of the disclosure has been described in detail, but the disclosure is not limited to this embodiment. The disclosure may include, for example, variations described below. 
     (1) The environment sensor  97  is not limited to the detection of the temperature or humidity around the intermediate transfer belt  141  inside the image forming apparatus  10 . In a variation, the environment sensor may detect the temperature or humidity of an environment around an installation site of the image forming apparatus  10 . 
     (2) Furthermore, in the embodiment, a change (an increase) in the resistance value of the intermediate transfer belt  141  is detected based on the whitening of the belt surface, but the disclosure is not limited to this. The disclosure includes the belt surface the color of which is changed to another color depending on the material composition of the intermediate transfer belt  141  when attachment adheres to the belt surface. The density threshold D0 may be set according to the change in color. Additionally, the disclosure is not limited to the detection of a change in the resistance value of the intermediate transfer belt  141  based on the density of the belt surface. Another resistance detecting section (not shown in the drawings) may directly detect the resistance value of the intermediate transfer belt  141 . 
     (3) Furthermore, in the embodiment, the following aspect has been described. That is, when the image forming apparatus  10  is powered on, the speed adjusting section  92  determines whether or not to adjust the speed ratio Vb/Vd between the circumferential rotating speed Vb of the intermediate transfer belt  141  and the rotation speed Vd of the photosensitive drum  20 . However, the disclosure is not limited to this aspect. The speed ratio Vb/Vd may be adjusted when each print job for the image forming apparatus  10  is started or when an image quality adjusting operation is performed. 
     (4) Additionally, in the embodiment, the following aspect has been described with reference to  FIG. 6 . That is, when the detected temperature T is lower than the temperature threshold T0 (YES in step S 002 ), the speed adjusting section  92  determines that the high temperature control need not be performed on the speed ratio Vb/Vd. The disclosure is not limited to this aspect. After determining that the high temperature control need not be performed on the speed ratio Vb/Vd, the speed adjusting section  92  may further check the belt for whitening as in the case of step S 004  and adjust the speed ratio Vb/Vd according to the result of the check. 
     Although the present disclosure has been fully described by way of example with reference to the accompanying drawings, it is to be understood that various changes and modifications will be apparent to those skilled in the art. Therefore, unless otherwise such changes and modifications depart from the scope of the present disclosure hereinafter defined, they should be construed as being included therein.