Patent Publication Number: US-9885982-B2

Title: Transfer device and image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2016-006413 filed Jan. 15, 2016. 
     BACKGROUND 
     (i) Technical Field 
     The present invention relates to a transfer device and an image forming apparatus. 
     (ii) Related Art 
     In a transfer device including a second transfer roller in which a surface layer is in tight contact with an outer side of an elastic layer without being bonded, when a scraping member touches a surface of the second transfer roller, so-called filming, in which a toner component thinly adheres to the surface layer on the outer side of the elastic layer in the second transfer roller, is sometimes caused by waviness of the surface layer. 
     SUMMARY 
     According to an aspect of the invention, there is provided a transfer device including a second transfer roller that has an elastic layer and a surface layer disposed in tight contact with an outer side of the elastic layer and rotates to transfer a toner image on a surface of an intermediate transfer body onto a recording medium, a scraping member that is in contact with the second transfer roller to scrape an attached substance off the surface layer, and a pressed member that is pressed against the surface layer on a downstream side of a contact portion with the intermediate transfer body and on an upstream side of the scraping member in a rotating direction of the second transfer roller and rotates in a forward direction with respect to rotation of the second transfer roller. 
    
    
     
       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 structural view of an image forming apparatus including a transfer device according to a first exemplary embodiment of the present invention; 
         FIG. 2  is a cross-sectional view of a second transfer roller used in the image forming apparatus illustrated in  FIG. 1 , taken along a direction orthogonal to an axial direction; 
         FIG. 3  is a structural view of the transfer device used in the image forming apparatus illustrated in  FIG. 1 ; 
         FIG. 4  is a structural view of the surroundings of a pressed member in the transfer device used in the image forming apparatus illustrated in  FIG. 3 ; 
         FIG. 5  is a structural view of a transfer device according to a second exemplary embodiment of the present invention; 
         FIG. 6  is a structural view of a transfer device according to a third exemplary embodiment of the present invention; 
         FIG. 7  is a graph showing the relationship between the maximum wave height of the surface layer of the second transfer roller and the toner slip amount; 
         FIG. 8  is a plan view illustrating a print pattern in which a part of a toner image on an intermediated transfer belt is transferred on a region of a recording medium including edges; and 
         FIG. 9  is a table comparing the occurrence state of filming on the surface of the second transfer roller between an example in which a pressed member is provided to be pressed against the second transfer roller and a comparative example in which the pressed member to be pressed against the second transfer roller is not provided. 
     
    
    
     DETAILED DESCRIPTION 
     Image forming apparatuses according to exemplary embodiments of the present invention will be described below with reference to the drawings. 
       FIG. 1  is a structural view of an image forming apparatus according to a first exemplary embodiment, that is, a so-called tandem image forming apparatus  10 . 
     As illustrated in  FIG. 1 , in the image forming apparatus  10 , image forming units  22  for four colors of yellow, magenta, cyan, and black (specifically, image forming units  22   a ,  22   b ,  22   c , and  22   d ) are provided inside a body housing  21 , and a belt module  23  disposed along an arrangement direction of the image forming units  22  is provided above the image forming units  22 . Further, in the image forming apparatus  10 , a cassette  24  in which recording media such as paper (not illustrated) are stored is provided in a lower part of the body housing  21 , and a transport path  25  through which the recording media are transferred extends upward from the cassette  24 . 
     For example, the image forming units  22  form toner images of yellow, magenta, cyan, and black in this order from the upstream side in the circling direction of an intermediate transfer belt  80  (the arrangement order is not always limited to this order). The image forming units  22  include their respective photoconductor units  30  and developing units  33  and one common exposure unit  40 . Each of the photoconductor units  30  includes a photoconductor drum  31 , a charging roller  32  that charges the photoconductor drum  31 , and a cleaning device  34  that removes residual toner from the photoconductor drum  31 . The exposure unit  40  stores, inside a unit case  41 , for example, four semiconductor lasers (not illustrated), one polygonal mirror  42 , an imaging lens (not illustrated), and mirrors (not illustrated) each corresponding to one of the photoconductor units  30 . Each of the developing units  33  develops an electrostatic latent image formed on the photoconductor drum  31  by exposure by the exposure unit  40  with corresponding color toner (for example, having a negative polarity). In an upper part of the body housing  21 , toner cartridges  35  (specifically, toner cartridges  35   a ,  35   b ,  35   c , and  35   d ) are provided to supply color component toners to the developing units  33 . 
     The belt module  23  is structured by stretching an intermediate transfer belt  80  serving as an example of an intermediate transfer body between a pair of support rollers  81  and  82  (one of them is a driving roller). First transfer rollers  51  are disposed on a back surface of the intermediate transfer belt  80  correspondingly to the photoconductor drums  31  of the photoconductor units  30 . By applying a voltage having a polarity opposite from the toner charging polarity to the first transfer rollers  51 , toner images on the photoconductor drums  31  are electrostatically transferred onto the intermediate transfer belt  80 . Further, a transfer device  52  that forms a transfer unit is disposed at a position corresponding to the support roller  82  on the downstream side of the image forming unit  22   d  provided on the most downstream side of the intermediate transfer belt  80 , and second-transfers (collectively transfers) the toner images on the surface of the intermediate transfer belt  80  onto a recording medium. 
     The transfer device  52  includes a second transfer roller  84  disposed in pressure contact with a toner-image bearing surface of the intermediate transfer belt  80  and a back roller disposed on the back side of the intermediate transfer belt  80  and serving as an opposed electrode for the second transfer roller  84  (this roller also functions as the support roller  82  in the first exemplary embodiment). For example, the second transfer roller  84  is grounded, and a bias having the same polarity as the toner charging polarity is applied to the back roller (support roller  82 ). 
     A belt cleaning device  53  is disposed on the upstream side of the image forming unit  22   a  provided on the most upstream side of the intermediate transfer belt  80 , and removes residual toner from the intermediate transfer belt  80  with a cleaning blade  54 . 
     The cassette  24  is provided with a feeding roller  61  that feeds out recording media. A transport roller  62  that transports the recording media is disposed just downstream of the feeding roller  61 , and a registration roller  63  that supplies the recording media to a second transfer portion (transfer unit) at a predetermined timing is disposed in the transport path  25  located just upstream of the second transfer portion. A fixing device  66  is provided in the transport path  25  downstream of the second transfer portion, and an output roller  67  is provided downstream of the fixing device  66 . This output roller  67  outputs the recording media into a paper output section  68  in the upper part of the body housing  21 . 
     A manual supply device  71  is provided on a side of the body housing  21 . A recording medium on the manual supply device  71  is transported toward the transport path  25  by a feeding roller  72  and the transport roller  62 . Further, the body housing  21  is provided with a duplex recording unit  73 . When a duplex mode for recording images on both surfaces of a recording medium is selected, the duplex recording unit  73  reverses a recording medium having one recorded surface by the output roller  67 , takes in the recording medium by a guide roller  74  before the entrance, transports the recording medium along an internal recording-medium return transport path  76  by transport rollers  77 , and supplies the recording medium toward the registration roller  63  again. 
     Next, the transfer device  52  disposed inside the image forming apparatus  10  will be described. 
       FIG. 2  is a cross-sectional view of the second transfer roller  84  used in the transfer device  52  of the first exemplary embodiment, taken along a direction orthogonal to the axial direction. As illustrated in  FIG. 2 , the second transfer roller  84  includes a shaft portion  100  disposed in the longitudinal direction, an elastic layer  102  provided around the shaft portion  100 , and a surface layer  104  disposed in tight contact with an outer side of the elastic layer  102 . An adhesive layer is not provided between the surface layer  104  and the elastic layer  102 , and the surface layer  104  and the elastic layer  102  are not bonded to each other. The second transfer roller  84  transfers a toner image on the surface of the intermediate transfer belt  80  onto a recording medium when the recording medium is transported to a contact portion between the second transfer roller  84  and the intermediate transfer belt  80 . 
     Although not illustrated, the axial length of the shaft portion  100  is set to be larger than the axial length of the elastic layer  102  and the surface layer  104 . Both axial end portions of the shaft portion  100  are supported by unillustrated bearings so that the second transfer roller  84  rotates in a direction of arrow A (see  FIG. 3 ). The second transfer roller  84  may rotate to follow movement of the intermediate transfer belt  80 , or may be independently rotated in the direction of arrow A. 
     The elastic layer  102  is formed of a material that is softer (elastic modulus is lower) than the shaft portion  100 , for example, foamed resin. In the first exemplary embodiment, the elastic layer  102  is formed of conductive foamed polyurethane, and the thickness thereof is set at, for example, about 4 mm. For example, the Asker C hardness of the elastic layer  102  is set at 30° to 40°, and preferably at 35°. The Asker C hardness is measured by pressing a measurement needle of an Asker C hardness meter (manufactured by Kobunshi Keiki Co., Ltd.) against a surface of a measurement sheet of a thickness of 3 mm serving as a sample of the elastic layer  102  under conditions of 22° C. and 55% RH with a load of 0.5 kg. 
     The surface layer  104  is formed of a material that is harder than the elastic layer  102  and has a smooth surface. The surface layer  104  is formed by a resin tube (covering tube) that covers the elastic layer  102 . In the first exemplary embodiment, for example, the surface layer  104  is formed of conductive polyimide, and the thickness of the surface layer  104  is set at about 40 μm. 
     As a manufacturing method for the second transfer roller  84 , for example, a method for press-fitting an elastic roller having the elastic layer  102  into a covering tube that forms the surface layer  104  is adopted. 
       FIG. 3  illustrates the transfer device  52  of the first exemplary embodiment, and  FIG. 4  illustrates the surrounding of a pressed member  120  used in the transfer device  52 . As illustrated in  FIGS. 3 and 4 , the transfer device  52  includes the second transfer roller  84  and a cleaning member  110  serving as an example of a scraping member that is in contact with the surface of the second transfer roller  84  to scrape attached substances off the surface of the second transfer roller  84 . The transfer device  52  further includes a roll-shaped pressed member  120  that is pressed against the surface layer  104  on the downstream side of the contact portion with the intermediate transfer belt  80  and on the upstream side of the cleaning member  110  in the rotating direction of the second transfer roller  84 . 
     The cleaning member  110  includes a scraper  112  serving as an example of an abutting member that is in contact with the second transfer roller  84  with pressure to scrape attached substances off the surface of the second transfer roller  84 , and a support portion  114  that supports the scraper  112 . The support portion  114  is L-shaped in a side cross section. A root portion  112 A of the scraper  112  is fixed to one end portion of the support portion  114 , and the other end portion of the support portion  114  is fixed to a housing (not illustrated) of the transfer device  52 . The second transfer roller  84  rotates in the direction of arrow A, and a distal end portion  112 B of the scraper  112  is disposed in a posture such as to point toward the upstream side in the rotating direction of the second transfer roller  84 . 
     The scraper  112  is formed by a metallic platelike member. While the distal end portion  112 B of the scraper  112  in contact with the second transfer roller  84  has a pointed shape, the shape of the distal end portion  112 B of the scraper  112  may be changed. In the first exemplary embodiment, an etched product of SUS 304 (TA material) is used as the scraper  112 . The thickness of the scraper  112  is about 80 μm, and the length of a portion of the scraper  112  that is not restrained by the support portion  114  (length from a portion of the scraper  112  with no support portion  114  to the distal end portion  112 B) is 7.5 mm. In  FIGS. 3 and 4 , the thickness of the scraper  112  is larger than the actual one so that the structure of the scraper  112  is easily understood. 
     As illustrated in  FIG. 4 , the pressed member  120  includes a shaft portion  120 A and an outer peripheral portion  120 B provided on an outer side of the shaft portion  120 A. Both axial end portions of the shaft portion  120 A are supported by unillustrated bearings so that the pressed member  120  rotates in the forward direction (direction of arrow B) with respect to the rotation of the second transfer roller  84 . In other words, the pressed member  120  rotates in the same direction as the rotating direction of the second transfer roller  84  at the contact portion with the second transfer roller  84 . In the first exemplary embodiment, the pressed member  120  rotates in the forward direction (direction of arrow B) by following the rotation of the second transfer roller  84 . 
     Spring members  122  are provided at both axial end portions of the shaft portion  120 A in the pressed member  120 , and the pressed member  120  is biased toward the second transfer roller  84  by the spring members  122 . That is, the pressed member  120  is pressed against the surface layer  104  of the second transfer roller  84  by the spring members  122 . 
     In the transfer device  52  of the first exemplary embodiment, the pressed member  120  is disposed at the position on the upstream side of the cleaning member  110  in the rotating direction of the second transfer roller  84  and just before the contact portion of the surface layer  104  of the second transfer roller  84  with the cleaning member  110 . 
     In the first exemplary embodiment, the outer peripheral portion  120 B of the pressed member  120  is formed of, for example, synthetic resin. The material of the outer peripheral portion  120 B is not limited to synthetic resin, and, for example, the outer peripheral portion  120 B may be formed of metal, foamed resin, or a composite member obtained by combining two or more of metal, resin, and foamed resin. 
     In the transfer device  52 , the pressed member  120  is pressed against the surface layer  104  of the second transfer roller  84  on the downstream side of the contact portion with the intermediate transfer belt  80  and on the upstream side of the cleaning member  110  in the rotating direction of the second transfer roller  84 . Thus, the maximum wave height Wz of the surface layer  104  of the second transfer roller  84  is less than or equal to ½ of the toner volume average particle diameter at the contact portion of the cleaning member  110  with the surface layer  104  of the second transfer roller  84 . In the first exemplary embodiment, for example, the toner volume average particle diameter is 5 μm, and the maximum wave height Wz of the surface layer  104  of the second transfer roller  84  is 2.5 μm or less. More specifically, the maximum wave height Wz of the surface layer  104  is preferably within the range of 1.0 to 2.5 μm, more preferably within the range of 1.0 to 2.3 μm, and further preferably within the range of 1.0 to 2.0 μm. 
     Here, the maximum wave height Wz refers to the maximum height of a waviness curve of the surface layer  104 , and to the sum of the maximum peak height Zp and the maximum valley depth Zv of a contour curve in a reference length. The maximum wave height Wz is measured according to JIS-B0601&#39;2001. In the first exemplary embodiment, the filtered center waviness (filtered waviness curve) of the surface layer  104  is measured in a measurement length of 40 mm by using a surface texture and contour measuring instrument Surfcom (manufactured by Tokyo Seimitsu Co., Ltd.). The maximum wave height Wz is measured at plural portions on the surface layer  104  in the axial direction, and the average value of the maximum wave heights is calculated. 
     A particle distribution measuring device (Coulter Multisizer II: manufactured by Beckman Coulter, Inc.) is used as a device for measuring the toner volume average particle diameter, and the particle diameter is measured using ISOTON-II (manufactured by Beckman Coulter, Inc.) as an electrolyte. As a measurement method, 0.5 to 50 mg of a measurement sample is added to 2 ml of a 5% water solution of a surface-active agent, preferably sodium alkylbenzene sulfonate, as a dispersant, and the mixture is added to 100 to 150 ml of electrolyte. This electrolyte in which the measurement sample is suspended is subjected to dispersion for about one minute with an ultrasonic disperser, and the particle size distribution is measured with Caulter Multisizer II by using an aperture having an aperture diameter of 100 μm. The number of particles to be measured is 50000. A cumulative distribution of the measured particle size distribution is obtained from the small-diameter side in divided particle size ranges (channels), the particle size at a cumulative volume of 50% is defined as a volume average particle diameter D50v, and D50v is taken as the volume average particle diameter. 
     In the image forming apparatus  10  of the first exemplary embodiment, for example, so-called marginless printing is performed to transfer a part of a toner image on the surface of the intermediate transfer belt  80  onto a region of a recording medium including edges by the second transfer roller  84  (see  FIG. 8 ). Instead of marginless printing, normal printing may be performed so that the toner image on the surface of the intermediate transfer belt  80  is transferred by the second transfer roller  84  onto a region of the recording medium including no edges. 
     Next, a transfer device according to a second exemplary embodiment of the present invention will be described with reference to  FIG. 5 . The same components as those adopted in the above-described first exemplary embodiment are denoted by the same reference numerals, and descriptions thereof are skipped. 
     As illustrated in  FIG. 5 , a transfer device  130  includes, instead of the pressed member  120  of the first exemplary embodiment (see  FIG. 3 ), a roll-shaped pressed member  132  that is pressed against a surface layer  104  of a second transfer roller  84  on the downstream side of a contact portion with an intermediate transfer belt  80  (not illustrated) and on the upstream side of a cleaning member  110  in the rotating direction of the second transfer roller  84 . Further, the transfer device  130  includes a motor  134  serving as an example of a driving unit that drives the pressed member  132  in the forward direction (direction of arrow B) with respect to rotation of the second transfer roller  84 . 
     Next, a transfer device according to a third exemplary embodiment of the present invention will be described with reference to  FIG. 6 . The same components as those adopted in the above-described first and second exemplary embodiments are denoted by the same reference numerals, and descriptions thereof are skipped. 
     As illustrated in  FIG. 6 , a transfer device  150  includes, instead of the cleaning member  110  of the first exemplary embodiment (see  FIG. 3 ), a cleaning member  152  serving as an example of a scraping member that is in contact with a surface of a second transfer roller  84  to scrape attached substances off the surface of the second transfer roller  84 . 
     The cleaning member  152  includes a cleaning blade  154  that is in contact with the second transfer roller  84  with pressure (in an elastically deformed state) to scrape attached substances off the surface of the second transfer roller  84 , and a support portion  156  that supports the cleaning blade  154 . The support portion  156  is L-shaped in a side cross section. A root portion  154 A of the cleaning blade  154  is fixed to one end portion of the support portion  156 , and the other end portion of the support portion  156  is fixed to a housing (not illustrated) of the transfer device  150 . The second transfer roller  84  rotates in a direction of arrow A, and a distal end portion  154 B of the cleaning blade  154  is disposed in a posture such as to point toward the upstream side in the rotating direction of the second transfer roller  84 . 
     For example, the cleaning blade  154  is formed by a rectangular two-layer blade, and includes a cleaning layer in contact with the second transfer roller  84  and a back layer disposed on the back side of the cleaning layer. In the cleaning blade  154  of the third exemplary embodiment, the cleaning layer in contact with the second transfer roller  84  is formed by a rubber member having a thickness of about 0.5 mm, and the back layer is formed by a rubber member having a thickness of about 1.5 mm and having a hardness lower than that of the cleaning layer. The cleaning blade  154  does not always need to be composed of two layers, and, for example, may be formed by a single layer. 
     While the specific exemplary embodiments of the present invention have been described in detail, it is obvious to those skilled in the art that the present invention is not limited to these exemplary embodiments and that various exemplary embodiments can be adopted within the scope of the invention. 
     EXAMPLES 
     First Example 
     Next, a description will be given of an experiment conducted to investigate the slip amount of toner that passes through the cleaning member while changing the maximum wave height Wz of a surface layer  104  of a second transfer roller  84  in an image forming apparatus  10  of a first example. 
     In the image forming apparatus  10  of the first example, a mat image is transferred onto a recording medium P by the second transfer roller  84 , the image forming apparatus  10  is stopped at a time when the mat image passes through a cleaning member  110  (or cleaning member  152 ) in a first transfer cycle of the second transfer roller  84 , and the state of the surface layer  104  of the second transfer roller  84  (toner slip amount) at the position where the mat image passes through the cleaning member  110  (or cleaning member  152 ) is observed. 
     In this experiment, the maximum wave height Wz of the surface layer  104  of the second transfer roller  84  is changed by changing the condition for pressing a pressed member  120  against the second transfer roller  84 . 
     As a method for measuring the maximum wave height Wz, after the second transfer roller  84  passes through the pressed portion of the pressed member  120 , the maximum wave height Wz of the surface layer  104  is measured at the contact position where the second transfer roller  84  touches the cleaning member  110  (or cleaning member  152 ). Specifically, in a state in which the pressed member  120  is set on the second transfer roller  84 , the cleaning member  110  (or cleaning member  152 ) is removed, and the maximum wave height Wz of the surface layer  104  at the position where the second transfer roller  84  touches the cleaning member  110  (or cleaning member  152 ) is measured. The maximum wave height Wz is measured according to JIS-B0601&#39;2001. In the first example, the filtered center waviness (filtered waviness curve) of the surface layer  104  is measured with the surface texture and contour measuring instrument Surfcom (manufactured by Tokyo Seimitsu Co., Ltd.), and the measurement length is 40 mm. 
     As the toner, toner having a volume average particle diameter of 5 μm is used. As described above, the volume average particle diameter of the toner is measured with the particle distribution measuring device (Coulter Multisizer II: manufactured by Beckman Coulter, Inc.) by using ISOTON-II (manufactured by Beckman Coulter, Inc.) as an electrolyte. 
     The maximum wave height Wz of the surface layer  104  of the second transfer roller  84  is measured for a case in which cleaning member  152  having the cleaning blade  154  is used as the cleaning member and a case in which the cleaning member  110  having the scraper  112  is used as the cleaning member. 
       FIG. 7  is a graph showing the relationship between the maximum wave height Wz (μm) of the surface layer  104  of the second transfer roller  84  and the toner slip amount (particles/cm 2 ). As shown in  FIG. 7 , when the maximum wave height Wz of the surface layer  104  of the second transfer roller  84  is 2.5 μm or less, toner slipping from the cleaning members  110  and  154  on the surface of the second transfer roller  84  is suppressed. In  FIG. 7 , when the maximum wave height Wz of the surface layer  104  of the second transfer roller  84  is 4.3 μm, the pressed member  120  is not provided (the pressed member  120  is not in contact with the second transfer roller  84 ). That is, when the maximum wave height Wz of the surface layer  104  is less than or equal to about ½ of the volume average particle diameter of the toner, the toner slip amount before the method of the first example is performed (when the pressed member  120  is not provided) is reduced by about 80%. When the maximum wave height Wz of the surface layer  104  is 2.5 μm or less, the toner slip amount is substantially equal to when filming does not occur and the maximum wave height Wz of the surface layer  104  is 1.0 μm. 
     Second Example 
     A description will be given of an experiment in which, after ten thousand A4-sized recording media P are printed in a print pattern illustrated in  FIG. 8  by an image forming apparatus  10  of a second example, the surface of a second transfer roller  84  is observed to check the filming state. 
     As illustrated in  FIG. 8 , in the second example, an L-shaped toner image T is formed on an intermediate transfer belt  80  (see  FIG. 1 ), and a part T 1  of the toner image T on the intermediate transfer belt  80  is transferred on a region of a recording medium (paper in the second example) P including edges (marginless printing). A toner image T 2  is not on the recording medium P, and the toner image T 2  not on the recording medium P adheres to the second transfer roller  84 . 
     As a cleaning member  152  for cleaning the surface of the second transfer roller  84  (see  FIG. 6 ), a cleaning blade  154  formed by a two-layer blade is used. In the cleaning blade  154 , a cleaning layer in contact with the second transfer roller  84  is formed by a rubber member having a thickness of about 0.5 mm, and a back layer is formed by a rubber member having a thickness of about 1.5 mm and having a hardness less than that of the cleaning layer. 
     In the second example, the surface of the second transfer roller  84  is observed to check the filming state in the structure in which the pressed member  120  is pressed against the surface of the second transfer roller  84 . Further, as a comparative example, the surface of the second transfer roller  84  is observed to check the filming state in a structure in which the pressed member  120  is not provided on the surface of the second transfer roller  84 . 
     As the pressed member  120 , a roll-shaped pressed member  120  having a diameter of 2 mm and made of metal (SUS in the second example) is used, the biting amount of the pressed member  120  in the second transfer roller  84  is 0.8 mm, and the distance to the measurement portion (contact portion of the cleaning member  152 ) is 1.0 mm. 
     In the structure of the second example (in which the pressed member  120  is pressed against the surface of the second transfer roller  84 ), the maximum wave height Wz of the surface layer  104  is 1.33 μm at the position where the second transfer roller  84  is in contact with the cleaning member  152 . In the structure of the comparative example (in which the pressed member  120  is not provided on the surface of the second transfer roller  84 ), the maximum wave height Wz of the surface layer  104  is 4.32 μm at the position where the second transfer roller  84  is in contact with the cleaning member  152 . The maximum wave height Wz is measured according to JIS-B0601&#39;2001. In the second example, the filtered center waviness (filtered waviness curve) of the surface layer  104  is measured with the surface texture and contour measuring instrument Surfcom (manufactured by Tokyo Seimitsu Co., Ltd.), and the measurement length is 40 mm. 
     In the comparative example, marginless printing is performed under the same print condition as that of the second example, and the occurrence state of filming on the surface of the second transfer roller is checked. 
       FIG. 9  shows the occurrence states of filming on the surface of the second transfer roller  84  in the second example and the comparative example. In  FIG. 9 , “IMAGE FORMED” in “PAPER PASSING PART” corresponds to a portion where the part T 1  of the toner image T illustrated in  FIG. 8  is transferred on the recording medium P, and “IMAGE FORMED” in “PAPER NON-PASSING PART” corresponds to a portion where the toner image T 2  which is not on the recording medium P in  FIG. 8  is transferred. The occurrence state of filming is evaluated in six grades G 0  to G 5  from a state in which no filming occurs to a state in which much filming occurs through a state in which little filming occurs. 
     As shown in  FIG. 9 , in the image forming apparatus of the second example, it is confirmed that the occurrence state of filming on the surface of the second transfer roller  84  is G 0 , that is, good (◯ in  FIG. 9 ). In contrast, in the image forming apparatus of the comparative example, it is confirmed that the occurrence state of filming on the surface of the second transfer roller  84  is G 4 , that is, the filming occurrence state becomes worse than in the second example (x in  FIG. 9 ).