Patent Publication Number: US-8532547-B2

Title: Transfer belt unit and image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application Ser. No. 11/683,578 filed Mar. 8, 2007, now U.S. Pat. No. 8,233,829 and claims priority to Japanese application No. 2006-088482 filed on Mar. 28, 2006, the entire contents of each of which are incorporated herein by reference in their entirety. 
    
    
     BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a transfer belt unit that includes a transfer belt and an image forming apparatus that includes the transfer belt unit. 
     2. Description of the Related Art 
     An electrophotographic image forming apparatus has been used in which a toner image formed on a photoconductor as an image carrier is primarily transferred onto an endless transfer belt, and the transferred toner image is carried by endless movement of the transfer belt, and secondarily transferred onto a recording medium. 
     In such an image forming apparatus, it is necessary to apply a tension to the transfer belt to endlessly move the transfer belt in a tensioned state, so that a toner image primarily transferred onto the transfer belt can be secondarily transferred onto the recording medium accurately. However, if the tension is applied at all times to the transfer belt, deformation of a fixing belt may occur in a portion with a small curvature factor when the apparatus stops. The deformation is referred to as curling. 
     Japanese Patent Application Laid-open No. S61-130972 discloses an apparatus including a mechanism that applies an appropriate tension to the transfer belt at the time of driving the apparatus, and loosening the tension applied to the transfer belt at the time of stopping the apparatus. By loosening the tension at the time of stopping the apparatus, it is possible to prevent curling that occurs in a portion with a small curvature factor when the apparatus stops. 
     However, a mechanism that adjusts the tension automatically is required to apply the appropriate tension at the time of driving the apparatus, and loosening the tension at the time of stopping the apparatus, which makes the apparatus complicated, and causes a cost increase. 
     Through extensive researches by the inventor of the present invention, it has been found that curling likely occurs as a tensile elastic stress of material of the transfer belt increases, and the occurrence of curling can be prevented by using a material having a lower tensile elastic stress without a mechanism for adjusting the tension. 
     If, however, a material having a lower tensile elastic stress is used for the transfer belt, cracking at the end of the belt or belt waving may occur depending on the settings. The cracking at the end of the belt means that an end of the transfer belt cracks because the end in the width direction of the transfer belt comes in contact with a regulating member that regulates the movement in an axial direction of the belt at an axial end of at least one of a plurality of stretching rollers. The belt waving means that the end of the belt comes in contact with the regulating member and deforms, which causes the transfer belt to become wavy on the downstream side from the contact portion in an endless moving direction. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to at least partially solve the problems in the conventional technology. 
     According to an aspect of the present invention, a transfer belt unit includes a transfer belt that is endless and transfers a toner image transferred from an image carrier onto a transfer medium, a secondary transfer roller that is located to form a nip with the transfer belt at a position where the toner image is transferred onto the transfer medium, a plurality of stretching rollers that stretches the transfer belt, a drive unit that transmits a rotational driving force to at least one of the stretching rollers, and a regulating member that regulates axial movement of the transfer belt at an axial end of at least one of the stretching rollers. The transfer belt has a thickness not less than 100 micrometers and not more than 200 micrometers, a tension not less than 80 N/m and not more than 180 N/m, and a tensile elastic modulus not less than 1000 megapascals and not more than 2000 megapascals. The tension is obtained by dividing a tension [N] applied in an endless moving direction of the transfer belt by a belt width [m] which is a length of the transfer belt in a direction perpendicular to the endless moving direction. The secondary transfer roller has an Asker C hardness not less than 35 degrees and not more than 50 degrees. The stretching rollers have an outer diameter not less than 6 millimeters. 
     According to another aspect of the present invention, an image forming apparatus includes an image forming unit and a transfer belt unit. The image forming unit includes a plurality of image carriers, a writing unit that writes a latent image on the image carriers, and a developing unit that forms a toner image from the latent image on the image carriers. The transfer belt unit includes a transfer belt that has an endless surface onto which the toner image is transferred from each of the image carriers at primary transfer positions corresponding to the image carriers, and transfers the toner image onto a transfer medium at a secondary transfer position, a secondary transfer roller that is located to form a nip with the transfer belt at the secondary transfer position, a plurality of stretching rollers that stretches the transfer belt, a drive unit that transmits a rotational driving force to at least one of the stretching rollers, and a regulating member that regulates axial movement of the transfer belt at an axial end of at least one of the stretching rollers. The transfer belt has a thickness not less than 100 micrometers and not more than 200 micrometers, a tension not less than 80 N/m and not more than 180 N/m, and a tensile elastic modulus not less than 1000 megapascals and not more than 2000 megapascals. The tension is obtained by dividing a tension [N] applied in an endless moving direction of the transfer belt by a belt width [m] which is a length of the transfer belt in a direction perpendicular to the endless moving direction. The secondary transfer roller has an Asker C hardness not less than 35 degrees and not more than 50 degrees. The stretching rollers have an outer diameter not less than 6 millimeters. 
     The above and other objects, features, advantages and technical and industrial significance of this invention will be better understood by reading the following detailed description of presently preferred embodiments of the invention, when considered in connection with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic block diagram of relevant parts of a printer according to an embodiment of the present invention; and 
         FIG. 2  is an enlarged schematic of periphery of a tension roller of a transfer unit shown in  FIG. 1 . 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Exemplary embodiments of the present invention are explained below in detail with reference to the accompanying drawings. 
       FIG. 1  is a schematic block diagram of an example of a printer  100  as an image forming apparatus according to an embodiment of the present invention. The printer  100  is an electrophotographic tandem image forming apparatus of an intermediate transfer system and includes four photoconductor drums as image carriers. 
     The printer  100  includes four process cartridges  10 Y,  10 M,  10 C, and  10 K for forming yellow, magenta, cyan, and black (Y, M, C, and K) toner image above an intermediate transfer belt  15 . The process cartridges use different Y, M, C, and K color toners as an image forming material. However, the process cartridges have an identical configuration except of the colors, and the cartridges are replaced when the toner in a development apparatus has been consumed, or a service life of a part forming the process cartridge  10  ends. These process cartridges  10  include a photoconductor drum  1 Y,  1 M,  1 C, or  1 K, respectively. Letters Y, C, M, and K added to reference numerals indicate yellow, cyan, magenta, and black, respectively. These photoconductor drums  1 Y,  1 M,  1 C, and  1 K are arranged so that a rotation axis thereof is in a horizontal direction, and the photoconductor drums face a back and forth direction of the apparatus (normal direction to the page in  FIG. 1 ). The photoconductor drums are also arranged so that the respective rotation axes are positioned on the same horizontal plane, and are parallel to each other. In the present embodiment, the respective photoconductor drums  1 Y,  1 M,  1 C, and  1 K have a cylindrical shape having a diameter of 24 millimeters and are set to rotate at a circumferential velocity of 120 mm/sec. 
     Chargers  2 Y,  2 M,  2 C, and  2 K for uniformly charging the surfaces of the photoconductor drums are respectively provided around the respective photoconductor drums  1 Y,  1 M,  1 C, and  1 K. The chargers  2 Y,  2 M,  2 C, and  2 K are contact type charging units that respectively charge the surfaces of the photoconductor drums by bringing a charging roller rotating with the surface of the photoconductor drum into contact with each other. However, a non-contact type charging unit using a charger can be used. In the present embodiment, a DC bias or a bias in which an AC bias is superimposed on the DC bias is applied to the chargers  2 Y,  2 M,  2 C, and  2 K by a high-voltage power source (not shown) to charge so that surface potential of the respective photoconductor drums  1 Y,  1 M,  1 C, and  1 K becomes −500 volts uniformly. 
     An exposure device (not shown) as a latent image forming unit is arranged vertically above the photoconductor drums  1 Y,  1 M,  1 C, and  1 K. The exposure device irradiates the photoconductor drums  1 Y,  1 M,  1 C, and  1 K with light beams  3 Y,  3 M,  3 C, and  3 K, respectively, according to image information to form an electrostatic latent image of each color on each of the photoconductor drums  1 Y,  1 M,  1 C, and  1 K. A laser beam scanner using a laser diode can be used as the exposure device. 
     A transfer unit  30 , i.e., a transfer belt unit including the intermediate transfer belt  15  as an endless transfer belt, is arranged vertically below the process cartridges  10 Y,  10 M,  10 C, and  10   k . The transfer unit  30  includes, in addition to the intermediate transfer belt  15 , a tension roller  20 , four primary-transfer bias rollers  5 Y,  5 M,  5 C, and  5 K as primary transfer rollers, a secondary-transfer opposing roller  21 , and a belt cleaning unit  33 . The transfer unit  30  is detachably formed relative to the printer  100 , thereby enabling replacement of consumable parts at the same time. 
     Developing units  4 Y,  4 M,  4 C, and  4 K that develop the electrostatic latent image respectively formed on the surface of the photoconductor drum are provided around the respective photoconductor drums  1 Y,  1 M,  1 C and  1 K. A toner in a developer carried on a developing roller as a developer carrier in the respective developing units  4 Y,  4 M,  4 C, and  4 K is shifted to the electrostatic latent image on the photoconductor drums  1 Y,  1 M,  1 C, and  1 K by applying a predetermined developing bias from the high-voltage power source (not shown) to the developing roller, thereby allowing the toner to adhere on the electrostatic latent image. Accordingly, a toner image corresponding to the electrostatic latent image is respectively formed on the photoconductor drums  1 Y,  1 M,  1 C, and  1 K. 180 grams of one-component developer is stored in the developing unit at an initial stage of use. 
     The toner images of respective colors on the respective photoconductor drums  1 Y,  1 M,  1 C, and  1 K respectively developed by the developing units  4 Y,  4 M,  4 C, and  4 K are primarily transferred onto the intermediate transfer belt  15 , which is an intermediate transfer member, and superposed on each other. The intermediate transfer belt  15  is spanned over a plurality of stretching rollers such as the secondary-transfer opposing roller  21  that forms a secondary transfer member, the primary-transfer bias rollers  5 Y,  5 M,  5 C, and  5 K that form a primary transfer member, and the tension roller  20 . In the embodiment, a rotational driving force from a driving source as a driving unit (not shown) is transmitted to the secondary-transfer opposing roller  21  to rotate the secondary-transfer opposing roller  21 , and hence the intermediate transfer belt  15  endlessly moves. That is, in the embodiment, the secondary-transfer opposing roller  21  is a driving roller of the intermediate transfer belt  15 . Other stretching rollers can be used as the driving roller. A roller  16  as a cleaning opposing roller is arranged to oppose the belt cleaning unit  33 . The respective rollers stretching the intermediate transfer belt  15  are supported by a side plate of the transfer unit  30  at opposite ends in the axial direction. 
     As the secondary-transfer opposing roller  21 , which is the driving roller, polyurethane rubber (thickness from 0.3 millimeter to 1 millimeter), a thin layer coating roller (thickness from 0.03 millimeter to 0.1 millimeter), or the like can be used. In the embodiment, a urethane coating roller (thickness of 0.05 millimeter and diameter of 20 millimeters) is used, which has a small diameter change due to temperature. 
       FIG. 2  is an enlarged view of periphery of the tension roller  20  of the transfer unit  30 . The tension roller  20  is an aluminum pipe having a diameter of 20 millimeters, and a collar  20   a  having a diameter of 24 millimeters is press-fitted at the opposite ends of the tension roller  20 . The collar  20   a  is a regulating member that prevents the intermediate transfer belt  15  moves in an axial direction of the tension roller  20  and meanders. 
     In the embodiment, the regulating member is provided only on the tension roller  20 . However, the regulating member can be provided on the secondary-transfer opposing roller  21  or on the other stretching rollers. 
     As a material used for the intermediate transfer belt  15 , a resin film endless belt can be used, in which a conductive material such as carbon black is dispersed in polyvinylidene difluoride (PVDF), ethylene-tetrafluoroethylene copolymer (ETFE), polyimide (PI), polycarbonate (PC), thermoplastic elastomer (TPE), or the like. In the embodiment, a belt having a single layer structure in which carbon black is added to TPE having a belt tensile elastic modulus of from 1000 megapascals to 2000 megapascals (tensile elastic modulus: measured in conformity with ISO R1184-1970, test piece: width of 15 millimeters and length of 150 millimeters, elastic stress rate: 1 mm/min, and distance between grippers: 100 millimeters), a thickness of from 100 micrometers to 200 micrometers, and a belt width of 230 millimeters. 
     As a resistance of the intermediate transfer belt  15 , it is desired that volume resistivity is in a range of from 10 8  to 10 11  Ω·cm, and surface resistivity is in a range of from 10 8  to 10 11 Ω/□ in an environment of 23° C. and 50% RH (both measured by HirestaUP MCP-HT450 by Mitsubishi Chemical Corporation, under a condition of applied voltage of 500 volts and application time of 10 seconds). If the volume resistivity and the surface resistivity of the intermediate transfer belt  15  exceed the range, the transfer bias needs to be increased, resulting in an increase in power cost. Further, because the intermediate transfer belt  15  is charged, a measure such as increasing a set voltage value is required on a downstream side of imaging. Therefore, a single power supply can be hardly used as the power supply for applying the voltage to the primary transfer member. This is because the charging potential of the intermediate transfer belt  15  increases due to application of the transfer bias, and self-discharge becomes difficult. As a measure against the disadvantage, a discharging mechanism for discharging the intermediate transfer belt  15  is required, which leads to a cost increase. On the other hand, if the volume resistivity and the surface resistivity fall below the above range, because the charging potential of the intermediate transfer belt  15  quickly attenuates, it is advantageous to discharge by self-discharge. However, because the transfer current flowing at the time of transfer likely flows in a surface direction, scattering of toner occurs. Accordingly, it is desired that the volume resistivity and the surface resistivity of the intermediate transfer belt  15  are in the above range. 
     There is an advantage by using TPE as the material for the intermediate transfer belt  15  in that a balance between the surface resistivity and the volume resistivity as the electrical resistance can be easily adjusted, while satisfying the range of the belt elastic modulus. Because the surface resistivity and the volume resistivity can be adjusted to a desired balance, excellent transfer can be performed. Further, because the adjustment is relatively easy, cost reduction can be achieved. 
     As the primary transfer member facing the photoconductor drums  1 Y,  1 M,  1 C, and  1 K with the intermediate transfer belt  15  therebetween, a conductive blade, a conductive sponge roller, or a metal roller can be used. In the embodiment, the primary-transfer bias rollers  5 Y,  5 M,  5 C, and  5 K made of metal having a diameter of 8 millimeters are used. The primary-transfer bias rollers  5 Y,  5 M,  5 C, and  5 K are offset relative to the photoconductor drums  1 Y,  1 M,  1 C, and  1 K by 8 millimeters in a moving direction of the intermediate transfer belt  15  and 1 millimeter vertically upwards. A transfer electric field is formed between the intermediate transfer belt  15  and the photoconductor drums  1 Y,  1 M,  1 C, and  1 K by commonly applying a predetermined primary transfer bias of from +500 to +1000 volts to the primary-transfer bias rollers  5 Y,  5 M,  5 C, and  5 K by a primary transfer power source (not shown), so that the toner image on the photoconductor is electrostatically transferred to the intermediate transfer belt  15 . 
     Each of cleaning devices  8 Y,  8 M,  8 C, and  8 K as an image carrier cleaning unit for removing residual toner after transfer remaining on the photoconductor drum after primary transfer is provided around each of the photoconductor drums  1 Y,  1 M,  1 C, and  1 K. The cleaning devices  8 Y,  8 M,  8 C, and  8 K include cleaning blade  6 Y,  6 M,  6 C, and  6 K as a removing member, and first waste toner-collecting units  7 Y,  7 M,  7 C, and  7 K, respectively. Each of the cleaning blades  6 Y,  6 M,  6 C, and  6 K abuts the back of each photoconductor to scrape and remove the residual toner on the surface of the photoconductor drum. The residual toners having removed by the cleaning blades  6 Y,  6 M,  6 C, and  6 K are collected by the first waste toner-collecting units  7 Y,  7 M,  7 C, and  7 K. 
     The toner image transferred on the intermediate transfer belt  15  is secondarily transferred onto transfer paper  22 , which is a recording medium transferred to a secondary transfer area, in the secondary transfer area between the belt portion wound around the secondary-transfer opposing roller  21  and a secondary-transfer bias roller  25  as a secondary transfer roller. The toner image on the intermediate transfer belt  15  is electrostatically transferred onto a recording material by applying a predetermined secondary transfer bias to the secondary-transfer bias roller  25  by a high voltage power supply (not shown). 
     The secondary transfer bias roller  25  is formed by covering a metal core made of SUS or the like with an elastic layer such as urethane processed to have a resistance of from 10 6  to 10 10 Ω by a conductive material. As materials thereof, an ion conductive roller (urethane+carbon dispersion, NBR, hydrin), an electron conductive roller (EPDM), and the like can be used. In the embodiment, a urethane roller as a foam roller having a diameter of 20 millimeters and an Asker C hardness of from 35 to 50 degrees is used. 
     Because a transfer current hardly flows when the resistance of the secondary-transfer bias roller  25  exceeds the above range, high voltage needs to be applied for obtaining necessary transferability, thereby increasing power cost. Because high voltage needs to be applied, discharge occurs in a gap in front of or behind the secondary transfer nip, and white spots due to the discharge appears on a halftone image. This phenomenon is noticeable in a low temperature and low humidity environment (for example, 10° C., 15% RH). 
     On the other hand, when the resistance of the secondary-transfer bias roller  25  falls below the above range, it is difficult to maintain excellent transferability both in an image area in which toner images of a plurality of colors present on the same image are superposed, and in a monochrome image area. This is because, since the resistance of the secondary-transfer bias roller  25  is low, if the secondary transfer bias is set to a relatively low voltage capable of obtaining an optimum transfer current for the monochrome image area, sufficient transfer current cannot be obtained for the color image area. On the contrary, if the secondary transfer bias is set to a relatively high voltage capable of obtaining an optimum transfer current for color image area, excessive transfer current flows to the monochrome image area, thereby decreasing transfer efficiency. 
     The resistance of the secondary-transfer bias roller  25  is calculated from a current value flowing at the time of applying a voltage of 1000 volts to between the core and a conductive metal plate in a state with a load of 4.9 Newtons being respectively applied to the opposite ends of the core (in total, 9.8 Newtons at the both ends), by installing the secondary-transfer bias roller  25  on the metal plate. 
     The transfer paper  22  is fed by a resist roller pair  24 , matched with the timing at which the end of the toner image on the surface of the intermediate transfer belt  15  reaches the secondary transfer position, and the toner image on the intermediate transfer belt  15  is transferred onto the transfer paper  22  by applying the predetermined secondary transfer bias by the high voltage power supply (not shown). The transfer paper  22  is separated from the intermediate transfer belt  15  due to a curvature factor of the secondary-transfer opposing roller  21 , and the transfer paper  22  is ejected after the toner image transferred onto the transfer paper  22  is fixed by a fuser  26  as a fixing unit. 
     The belt cleaning unit  33  as an intermediate transfer member-cleaning unit for removing the residual toner after transfer remaining on the intermediate transfer belt  15  after secondary transfer is arranged at a position facing the roller  16  with the intermediate transfer belt  15  therebetween. The belt cleaning unit  33  includes a cleaning blade  31  as a removing member and a second waste toner-collecting unit  32 . The cleaning blade  31  abuts against the surface of the intermediate transfer belt  15 , and scrapes and removes the residual toner on the intermediate transfer belt  15 . The residual toner removed by the cleaning blade  31  is collected by the second waste toner-collecting unit  32 , and carried to a waste toner container  34  via a toner carrier path (not shown) to be collected therein. 
     In the embodiment, there are a monochrome mode for forming an image of any one color of yellow, magenta, cyan, and black, a two-color mode for superposing any two colors of yellow, magenta, cyan, and black to form an image of two colors, a three-color mode for superposing any three colors of yellow, magenta, cyan, and black to form an image of three colors, and a full color mode for forming the four-color image described above, and these modes can be specified by an operation unit. 
     In the embodiment, the process speed at the time of fixing the toner image is changed according to the type of the transfer paper  22 . Specifically, when the transfer paper  22  having a basis weight of 100 g/m 2  or more is used, the process speed is set to half speed. Accordingly, because the transfer paper  22  passes through the fixing nip formed by a fixing roller pair  58  in the fuser  26  over twice the time of the normal process speed, fixity of the toner image can be secured. 
     The transfer unit  30  in the embodiment integrally supports the intermediate transfer belt  15 , the tension roller  20 , the primary-transfer bias rollers  5 Y,  5 M,  5 C, and  5 K, the secondary-transfer opposing roller  21 , and the belt cleaning unit  33 , and is detachably formed relative to the printer  100 , so that consumable parts can be replaced at the same time. The transfer unit  30  can have such a configuration that it also integrally supports the secondary-transfer bias roller  25  and is detachable relative to the printer  100 . 
     EXPERIMENT 
     The transfer belt unit described above and the image forming apparatus using the same were evaluated by changing the belt thickness, belt tension, belt elastic modulus, diameter of the primary transfer roller, diameter of the cleaning opposing roller, and hardness of the secondary-transfer bias roller. 
     &lt;Driving Test&gt; 
     The intermediate transfer belt was continuously driven at a process speed of 120 mm/s to check the occurrence of cracking at the end of the belt and the occurrence of belt waving. 
     The defect of the cracking at the end of the belt is defined as cracking equal to or more than 1 millimeter visually confirmed at the end of the belt, and waving is defined as unevenness equal to or more than 1 millimeter visually confirmed on the surface of the belt at a position about 30 millimeters from the end of the belt. Because it was obvious that the primary transfer roller and the belt-cleaning opposing roller had a shallow angle of contact as shown in  FIG. 1 , and the outer diameters of these rollers did not largely affect the driving test result, a metal roller having a diameter of 8 millimeters was used for the both rollers. 
     The result of the driving test is shown in Table 1. 
     
       
         
           
               
               
               
               
               
             
               
                   
                 TABLE 1 
               
             
            
               
                   
                   
               
               
                   
                   
                   
                 Tensile 
                   
               
               
                   
                 Belt 
                 Belt 
                 elastic 
               
               
                   
                 thickness 
                 tension 
                 modulus 
                 Result 
               
            
           
           
               
               
               
               
               
               
            
               
                   
                 (μm) 
                 (N/m) 
                 (MPa) 
                 Cracking 
                 Waving 
               
               
                   
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Example 1 
                 150 
                 130 
                 1500 
                 ◯ 
                 ◯ 
               
               
                 Example 2 
                 100 
                 130 
                 1500 
                 Δ 
                 ◯ 
               
               
                 Example 3 
                 200 
                 130 
                 1500 
                 ◯ 
                 Δ 
               
               
                 Example 4 
                 150 
                 80 
                 1500 
                 ◯ 
                 Δ 
               
               
                 Example 5 
                 150 
                 180 
                 1500 
                 Δ 
                 ◯ 
               
               
                 Example 6 
                 150 
                 130 
                 1000 
                 ◯ 
                 Δ 
               
               
                 Example 7 
                 150 
                 130 
                 2000 
                 Δ 
                 ◯ 
               
               
                 Comparative 
                 80 
                 130 
                 1500 
                 XX 
                 — 
               
               
                 Example 1 
               
               
                 Comparative 
                 250 
                 130 
                 1500 
                 — 
                 XX 
               
               
                 Example 2 
               
               
                 Comparative 
                 150 
                 60 
                 1500 
                 — 
                 X 
               
               
                 Example 3 
               
               
                 Comparative 
                 150 
                 200 
                 1500 
                 X 
                 — 
               
               
                 Example 4 
               
               
                 Comparative 
                 150 
                 130 
                 500 
                 — 
                 XX 
               
               
                 Example 5 
               
               
                 Comparative 
                 150 
                 130 
                 2500 
                 X 
                 — 
               
               
                 Example 6 
               
               
                   
               
            
           
         
       
     
     In Table 1, a transfer belt unit that suffered a defect within 50 hours of driving is indicated by “xx”, the one that suffered a defect during 50 to 150 hours of driving is indicated by “x”, the one that suffered a defect during 150 to 300 hours of driving is indicated by “Δ”, the one that suffered no defect in 300 hours is indicated by “o”, and the one that was not evaluated due to suspension of the driving evaluation is indicated by “-”. 
     From Examples 1, 2, and 3, and Comparative Examples 1 and 2 in Table 1, it is seen that waving likely occurs as the thickness of the belt becomes thick, and on the contrary, cracking at the end of the belt likely occurs as the thickness of the belt becomes thin. This is because, as the thickness of the belt becomes thicker, the tension acting on a unit of volume of the belt decreases, and as the thickness of the belt becomes thinner, the tension acting on a unit of volume of the belt increases, in a state with the same tension being applied. It is conceived that, when the thickness of the belt is thick, and the tension acting on a unit of volume is small, the belt easily deforms in a relatively loose state, and when the belt comes in contact with the regulating member, the belt deforms to cause waving. On the other hand, when the thickness of the belt is thin, and the tension acting on a unit of volume is large, the belt hardly deforms in a relatively tensioned state, elastic deformation hardly occurs, and the belt is in a fragile state upon application of a force. Accordingly, when the belt comes in contact with the regulating member, the cracking at the end of the belt easily occurs. 
     From Examples 1, 4, and 5, and Comparative Examples 3 and 4 in Table 1, it is seen that waving easily occurs as the belt tension decreases, and on the contrary, the cracking at the end of the belt easily occurs as the belt tension increases. It is considered that this is because when the belt tension is small, the belt easily deforms in a relatively loose state, and when the belt comes in contact with the regulating member, the belt deforms to cause waving. On the other hand, when the belt tension is large, the belt hardly deforms in a relatively tensioned state, elastic deformation hardly occurs, and the belt is in a fragile state upon application of a force. Accordingly, when the belt comes in contact with the regulating member, the cracking at the end of the belt easily occurs. 
     From Examples 1, 6, and 7, and Comparative Examples 5 and 6 in Table 1, it is seen that waving easily occurs as the belt tensile elastic modulus decreases, and on the contrary, the cracking at the end of the belt easily occurs as the belt tensile elastic modulus increases. It is considered that this is because when the belt tensile elastic modulus is small, the belt easily deforms, and when the belt comes in contact with the regulating member, the belt deforms to cause waving. On the other hand, when the belt tensile elastic modulus is large, the belt hardly deforms and is in a fragile state, and when the belt comes in contact with the regulating member, the cracking at the end of the belt easily occurs. 
     &lt;Storage Test&gt; 
     As a storage test, after the image forming apparatus was left in an environment of 45° C. and 90% RH for two weeks, a single-color halftone image was printed at 600 dots per inch in an environment of 23° C. and 50% RH, and curling (lateral stripe) was checked. In the storage test, a transfer belt having a belt thickness of 150 micrometers, and a belt tensile elastic modulus of 1500 megapascals, which achieved an excellent result from the driving test for both “cracking” and “waving”, was used as the transfer belt, to simplify the experiments. 
     The result of the storage test is shown in Table 2. 
     
       
         
           
               
               
               
               
               
               
             
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                   
                 Diameter 
                   
                 Hardness 
                   
               
               
                   
                   
                 of 
                 Diameter 
                 of 
               
               
                   
                   
                 primary- 
                 of 
                 secondary- 
               
               
                   
                   
                 transfer 
                 cleaning 
                 transfer 
               
               
                   
                 Belt 
                 bias 
                 opposing 
                 bias 
               
               
                   
                 tension 
                 roller 
                 roller 
                 roller 
               
               
                   
                 (N/m) 
                 (mm) 
                 (mm) 
                 (mm) 
                 Result 
               
               
                   
                   
               
             
            
               
                   
               
            
           
           
               
               
               
               
               
               
            
               
                 Example 1 
                 130 
                 8 
                 8 
                 42 
                 ◯ 
               
               
                 Example 4 
                 80 
                 8 
                 8 
                 42 
                 ◯ 
               
               
                 Example 5 
                 180 
                 8 
                 8 
                 42 
                 Δ 
               
               
                 Example 8 
                 130 
                 6 
                 6 
                 42 
                 Δ 
               
               
                 Example 9 
                 130 
                 6 
                 6 
                 35 
                 Δ 
               
               
                 Example 10 
                 130 
                 6 
                 6 
                 50 
                 Δ 
               
               
                 Comparative 
                 200 
                 8 
                 8 
                 42 
                 X 
               
               
                 Example 7 
               
               
                 Comparative 
                 130 
                 6 
                 6 
                 32 
                 X 
               
               
                 Example 8 
               
               
                 Comparative 
                 130 
                 6 
                 6 
                 52 
                 X 
               
               
                 Example 9 
               
               
                   
               
            
           
         
       
     
     In Table 2, an apparatus in which a stripe was visually confirmed in an image is indicated by “x”, the one in which curling was visually confirmed on the transfer belt but no stripe was visually seen in an image is indicated by “Δ”, and the one in which no stripe is seen both on an image and on the transfer belt is indicated by “o”. 
     From Examples 1, 4, and 5, and Comparative Example 7 in Table 2, it can be confirmed that curling likely occurs as the tension increases. 
     Further, from Examples 1 and 8 in Table 2, it can be confirmed that curling likely occurs as the roller diameter decreases. 
     From Examples 9 and 10 and Comparative Examples 8 and 9 in Table 2, it can be confirmed that even when the hardness of the secondary-transfer bias roller is too high or too low, a lateral stripe is generated on the image. If the secondary-transfer bias roller has high hardness, contactability between the roller and the belt decreases, and formation of the secondary transfer nip becomes unstable, thereby disturbing the image. In Comparative Example 9, curling of the transfer belt is sharply picked up, and results in a lateral stripe on the image. If the secondary-transfer bias roller has low hardness, the secondary-transfer bias roller itself deforms when being left as it is, thereby forming a lateral stripe on the image resulting from deformation of the secondary-transfer bias roller. 
     From these experiments, it has been found that all of curling of the belt, cracking at the end of the belt, and waving of the belt can be prevented by setting the thickness of the transfer belt in a range of 100 micrometers to 200 micrometers, belt tension in a range of 80 N/m to 180 N/m, belt tensile elastic modulus in a range of 1000 megapascals to 2000 megapascals, the outer diameter of all the rollers coming in contact with the belt at a belt contact portion to 6 millimeters or more, and the hardness of the secondary-transfer bias roller to Asker C hardness in a range of 35 degrees to 50 degrees. 
     Accordingly, the printer  100  as the image forming apparatus having high reliability can be provided by setting the intermediate transfer belt  15  and the secondary-transfer bias roller  25  as above. Further, in a configuration in which the transfer unit  30  integrally supports the secondary-transfer bias roller  25 , the transfer unit  30  as the transfer belt unit having high reliability can be provided by setting the intermediate transfer belt  15  and the secondary-transfer bias roller  25  as above. 
     As set forth hereinabove, according to an embodiment of the present invention, the occurrence of curling, cracking at the end of the belt, and waving of the belt can be reduced if the following conditions are satisfied: 
     The intermediate transfer belt has a belt thickness in a range from 100 micrometers to 200 micrometers, belt tension from 80 N/m to 180 N/m, and tensile elastic modulus from 1000 megapascals to 2000 megapascals. 
     The secondary transfer roller has an Asker C hardness of 35 degrees to 50 degrees. 
     The outer diameter of the stretching rollers for stretching the intermediate transfer belt is equal to or larger than 6 millimeters. 
     Thus, the image forming apparatus having high reliability can be realized. In the configuration in which the transfer unit integrally supports the secondary-transfer bias roller, the transfer unit as the transfer belt unit having high reliability can be realized. Additionally, an inexpensive intermediate transfer belt that satisfies the above belt tensile elastic modulus can be obtained by using thermoplastic elastomer (TPE) as the material of the intermediate transfer belt. 
     Moreover, by using metal rollers for the primary-transfer bias rollers of the stretching rollers, the manufacturing cost can be reduced as compared to an instance in which rollers having a resin layer is used. Besides, through the use of a urethane roller, i.e., a foam roller having an Asker C hardness of not less than 35 degrees and not more than 50 degrees, as a secondary transfer roller, a secondary transfer nip suitable for image formation can be formed. Thus, an excellent image can be obtained. 
     Furthermore, the primary-transfer bias rollers are arranged offset with respect to the photoconductor drums. Therefore, a primary transfer nip can be formed stably and primary transfer can be performed favorably, resulting in excellent image quality. 
     Although the invention has been described with respect to a specific embodiment for a complete and clear disclosure, the appended claims are not to be thus limited but are to be construed as embodying all modifications and alternative constructions that may occur to one skilled in the art that fairly fall within the basic teaching herein set forth.