Patent Publication Number: US-10324405-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. 2017-118000 filed Jun. 15, 2017. 
     BACKGROUND 
     Technical Field 
     The present invention relates to a transfer device and an image forming apparatus. 
     SUMMARY 
     A transfer device according to an aspect of the invention includes a holding body that holds a toner image, a transfer body that presses a recording medium against the holding body with a first pressing force or a second pressing force, which is smaller than the first pressing force, to transfer the toner image to the recording medium while transporting the recording medium between the transfer body and the holding body, and a setting portion that sets a pressing force exerted on the recording medium at the second pressing force when a mass of a toner of an uppermost toner layer constituting the toner image and disposed on the holding body is equal to or exceeds a threshold. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIGS. 1A and 1B  are sectional views of components including a second transfer portion and a transfer belt of an image forming apparatus according to a first exemplary embodiment of the present invention; 
         FIGS. 2A and 2B  are sectional views of components including a second transfer portion and a transfer belt of an image forming apparatus according to the first exemplary embodiment of the present invention; 
         FIG. 3  is a diagram of a structure of a toner layer forming portion of the image forming apparatus according to the first exemplary embodiment of the present invention; 
         FIG. 4  is a diagram of a structure of the toner layer forming portion and a transfer portion of the image forming apparatus according to the first exemplary embodiment of the present invention; 
         FIG. 5  is a schematic diagram of a structure of the image forming apparatus according to the first exemplary embodiment of the present invention; 
         FIGS. 6A and 6B  are schematic diagrams of a toner image obtained by superposing toner layers on a transfer belt using the image forming apparatus according to the first exemplary embodiment of the present invention; 
         FIGS. 7A and 7B  are schematic diagrams of a white toner and a color toner used in the image forming apparatus according to the first exemplary embodiment of the present invention; 
         FIGS. 8A and 8B  are schematic diagrams of a pigment of a white toner used in the image forming apparatus according to the first exemplary embodiment of the present invention; 
         FIGS. 9A and 9B  are schematic diagrams of a white toner used in the image forming apparatus according to the first exemplary embodiment of the present invention; 
         FIGS. 10A and 10B  are schematic diagrams of a pigment of a color toner used in the image forming apparatus according to the first exemplary embodiment of the present invention; 
         FIGS. 11A and 11B  are schematic diagrams of a color toner used in the image forming apparatus according to the first exemplary embodiment of the present invention; 
         FIGS. 12A and 12B  are tables showing the evaluation results relating to the image forming apparatus according to the first exemplary embodiment of the present invention and the evaluation results relating to an image forming apparatus according to a comparative example; 
         FIG. 13  is a graph of the evaluation results of a white toner and color toners used in the image forming apparatus according to a first exemplary embodiment of the present invention; 
         FIG. 14  is a graph showing the relationship between an amount of depression and the bendability of a recording medium used in the image forming apparatus according to the first exemplary embodiment of the present invention; 
         FIG. 15  is a graph showing the relationship between an amount of depression and an amount of electric discharge of a transfer portion used in the image forming apparatus according to the first exemplary embodiment of the present invention; 
         FIG. 16  is a schematic diagram of the evaluation results on an image forming apparatus according to a comparative example in comparison with the evaluation results on the image forming apparatus according to the first exemplary embodiment of the present invention in relation to toner scattering on a recording medium; 
         FIG. 17  is a sectional view of components including a second transfer portion and a transfer belt of an image forming apparatus according to a comparative example in comparison with the image forming apparatus according to the first exemplary embodiment of the present invention; 
         FIGS. 18A and 18B  are sectional views of components including a second transfer portion and a transfer belt of an image forming apparatus according to a comparative example in comparison with the image forming apparatus according to the first exemplary embodiment of the present invention; 
         FIGS. 19A and 19B  are sectional views of components including a second transfer portion and a transfer belt of an image forming apparatus according to a comparative example in comparison with the image forming apparatus according to the first exemplary embodiment of the present invention; 
         FIG. 20  is a diagram of a structure of a toner layer forming portion and a transfer portion of an image forming apparatus according to a second exemplary embodiment of the present invention; 
         FIG. 21  is a schematic diagram of a toner image obtained by superposing toner layers on a transfer belt using the image forming apparatus according to the second exemplary embodiment of the present invention; 
         FIG. 22  is a schematic diagram of a silver toner used in the image forming apparatus according to the second exemplary embodiment of the present invention; 
         FIGS. 23A and 23B  are schematic diagrams of a pigment of a silver toner used in the image forming apparatus according to the second exemplary embodiment of the present invention; 
         FIGS. 24A and 24B  are schematic diagrams of a silver toner used in the image forming apparatus according to the second exemplary embodiment of the present invention; 
         FIGS. 25A and 25B  are sectional views of components including a second transfer portion and a transfer belt of an image forming apparatus according to a third exemplary embodiment of the present invention; 
         FIGS. 26A and 26B  are sectional views of the components including a second transfer portion and a transfer belt of an image forming apparatus according to a fourth exemplary embodiment of the present invention; 
         FIG. 27  is a schematic diagram of the structure of the image forming apparatus according to the fourth exemplary embodiment of the present invention; 
         FIGS. 28A and 28B  are sectional views of the components including a second transfer portion and a transfer belt of an image forming apparatus according to a fifth exemplary embodiment of the present invention; and 
         FIG. 29  is a schematic diagram showing the range of a sheet member over which the sheet member is firmly pressed by a pressing portion, the sheet member being used in the image forming apparatus according to the fifth exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     First Exemplary Embodiment 
     Examples of a transfer device and an image forming apparatus according to a first exemplary embodiment of the present invention are described with reference to  FIGS. 1A to 19B . The arrow H shown in these drawings denotes the vertical direction and an apparatus height direction, the arrow W denotes the horizontal direction and an apparatus width direction, and the arrow D denotes the horizontal direction and an apparatus depth direction. 
     Entire Structure 
     As illustrated in  FIG. 5 , an image forming apparatus  10  includes an image forming unit  12 , which forms images by electrophotography, and a transport device  18 , which includes multiple transport rollers (not denoted with reference signs) that transport sheet members P (an example of recording media) along a transport path  16  of the sheet members P. 
     The image forming apparatus  10  includes a cooling portion  20 , which cools a sheet member P on which an image is formed, a correcting portion  22 , which corrects bending of a sheet member P, and an image inspecting portion  24 , which inspects an image formed on a sheet member P. 
     The image forming apparatus  10  also includes a reverse path  26 , which reverses a sheet member P having an image formed on its top surface and transports the sheet member P again toward the image forming unit  12  to form images on both surfaces of the sheet member P. 
     The image forming apparatus  10  having the above structure forms an image (toner image) formed by the image forming unit  12 , on the top surface of a sheet member P transported along the transport path  16 . The sheet member P having an image formed thereon passes through the cooling portion  20 , the correcting portion  22 , and the image inspecting portion  24  in this order and is discharged to the outside of the apparatus. 
     When an image is to be formed on the back surface of a sheet member P, a sheet member P having an image formed on its top surface is transported along the reverse path  26  and the image forming unit  12  forms an image again on the back surface of the sheet member P. 
     Image Forming Unit 
     The image forming unit  12  includes multiple toner layer forming portions  30 , which respectively form toner layers of various colors, a transfer belt  50 , which holds a toner image formed of one or more toner layers, and a transfer portion  14 , which transfers the toner image to a sheet member P. The image forming unit  12  also includes a setting portion  58  (see  FIG. 1A ), which sets a pressing force exerted to press the sheet member P against the transfer belt  50 , and a fixing device  34 , which fixes a toner image transferred to a sheet member P by the transfer portion  14  onto the sheet member P. 
     The multiple toner layer forming portions  30  form toner layers of different colors. In the present exemplary embodiment, the toner layer forming portions  30  are prepared for five colors of yellow (Y), magenta (M), cyan (C), black (K), and white (W). Reference characters Y, M, C, K, and W appended to the reference numerals in  FIG. 5  represent the above colors. In the present exemplary embodiment, yellow (Y), magenta (M), cyan (C), and black (K) are basic colors to output a color image. Two toner layer forming portions  30  are prepared for white (W). 
     In the following description, the characters Y, M, C, K, and W appended to the reference numerals are omitted unless yellow (Y), magenta (M), cyan (C), black (K), and white (W) need to be distinguished from each other. Hereinbelow, yellow (Y), magenta (M), cyan (C), and black (K) may be collectively referred to as “non-white colors”. 
     The toner layer forming portions  30  for various colors basically have the same structure except for using different color toners. As illustrated in  FIG. 3 , each toner layer forming portion  30  includes a rotating cylindrical image carrier  40 , and a charging device  42 , which charges the image carrier  40 . Each toner layer forming portion  30  also includes an exposure device  44 , which irradiates the charged image carrier  40  with exposure light to form an electrostatic latent image on the image carrier  40 , and a developing device  46 , which develops an electrostatic latent image with a developer G containing toner into a toner layer. Here, the developer G used in the present exemplary embodiment is a binary developer containing a toner and a carrier. 
     Each image carrier  40  for the corresponding color is grounded and touches the rotating transfer belt  50  (described in detail below). As illustrated in  FIG. 4 , the toner layer forming portions  30  for white (W), yellow (Y), magenta (M), cyan (C), black (K), and white (W) are arranged in this order in the horizontal direction from the upstream side in the direction in which the transfer belt  50  is rotated (see the arrow A in the drawing). 
     As illustrated in  FIG. 4 , the transfer portion  14  includes first transfer rollers  52 , which rotate and transfer toner layers formed on the image carriers  40  of the corresponding colors to the transfer belt  50 . The transfer portion  14  also includes a second transfer portion  54 , which transfers a toner image formed of one or more toner layers transferred to the transfer belt  50  onto a sheet member P. The transfer portion  14 , the transfer belt  50 , and the setting portion  58  are described in detail below. 
     As illustrated in  FIG. 5 , the fixing device  34  includes a fixing belt  60 , which is wound around multiple rollers (not denoted with reference signs) and heated, and a pressing roller  62 , which presses a sheet member P against the fixing belt  60 . 
     In this structure, the rotating fixing belt  60  and the pressing roller  62  transport a sheet member P to which a toner image has been transferred while holding the sheet member P therebetween, so as to fix the toner image to the sheet member P. 
     Structure of Related Portions 
     The following describes toners used in the developing device  46 , the transfer belt  50 , serving as an example of a holding body, the transfer portion  14 , serving as an example of a transfer body, and a setting portion  58 , which sets a pressing force exerted to press the sheet member P against the transfer belt  50 . The transfer belt  50 , the transfer portion  14 , and the setting portion  58  are included in a transfer device  38 . 
     Toners Used in Developing Device  46   
     The developing device  46 W employs a white toner (also referred to as “a W toner”, below)  200 , and the developing devices  46 Y,  46 M,  46 C, and  46 K employ color toners  300  for non-white colors. Now, the white toner  200  and the color toners  300  are described. 
     The white toner  200  is used on the sheet member P as a base coat for non-white colors. Specifically, a solid layer (solid image) of the white toner  200  is formed on a sheet member P as a base coat for non-white colors to enhance color reproduction of the toner image. 
     When the sheet member P, serving as a recording medium, is a paper medium, a W toner layer, a K toner layer, a C toner layer, a M toner layer, and a Y toner layer are superposed from top to bottom in this order on the sheet member P, which is a paper medium. When, on the other hand, the sheet member P serving as a recording medium is a transparent film, a K toner layer, a C toner layer, a M toner layer, a Y toner layer, and a W toner layer are superposed from top to bottom in this order on the sheet member P to allow an image to be viewed through the film. 
     White Toner  200   
     As illustrated in  FIG. 7A , the white toner  200  contains a spherical pigment  210  and a binding resin  220 . The spherical pigment  210  is formed from a titanium oxide (an example of a metallic oxide). The binding resin  220  is formed from a known resin material. The binding resin  220  is less electrically conductive than the spherical pigment  210 . 
     In the state where the spherical pigment  210  is placed on a flat surface  500 , a lateral dimension X 1  and a front-rear dimension Z 1  of the spherical pigment  210 , viewed from the top in  FIG. 8A , are equal to the lateral dimension X 1  and a vertical dimension Y 1  of the spherical pigment  210 , viewed from the side in  FIG. 8B . 
     The white toner  200  containing the spherical pigment  210  is also spherical in the same manner as the spherical pigment  210 . Thus, when the white toner  200  is placed on the flat surface  500 , a lateral dimension A 1  and a front-rear dimension B 1  of the white toner  200 , viewed from the top in  FIG. 9A , are equal to the lateral dimension A 1  and a vertical dimension C 1  of the white toner  200 , viewed from the side in  FIG. 9B . 
     The volumetric average particle diameter of the spherical pigment  210  or the white toner  200  is measured by using, for example, Coulter counter TAII (from Nikkaki Bios Co., Ltd.) or Multisizer II (from Nikkaki Bios Co., Ltd.). Specifically, within a particle range (channel) separated on the basis of the particle size distribution measured with this measuring instrument, the cumulative distribution is plotted from the smaller diameter with respect to the volume, and the particle diameter (D50v) of the cumulative percentage of 50% is used as a volumetric average particle diameter. Other volumetric average particle diameters below are measured similarly. 
     The standard volumetric average particle diameter of the spherical pigment  210  falls within a range of approximately 200 nm to 300 nm. The standard volumetric average particle diameter of the white toner  200  falls within a range of approximately 4 μm to 14 μm. 
     In the present exemplary embodiment, the volumetric average particle diameter of the white toner  200  is 8.5 μm, and the specific gravity of the white toner  200  is 1.6 g/cm 3 . Thus, the average mass (an example of mass) of the white toner  200  is 0.51×10 −9  g. 
     Color Toner  300   
     As illustrated in  FIG. 7B , each color toner  300  does not contain the spherical pigment  210 . The color toner  300  contains a pigment  310 , other than the spherical pigment  210 , and a binding resin  320 . The pigment  310  is formed of, for example, a nonmetal and nonmetallic oxide pigment (for example, an organic pigment). Specifically, the color toner  300  contains a pigment less electrically conductive than the spherical pigment  210 . The binding resin  320  is formed of a known resin material. 
     In the state where the spherical pigment  310  is placed on the flat surface  500 , a lateral dimension X 2  and a front-rear dimension Z 2  of the spherical pigment  310 , viewed from the top in  FIG. 10A , are equal to the lateral dimension X 2  and a vertical dimension Y 2  of the spherical pigment  310 , viewed from the side in  FIG. 10B . Specifically, the pigment  310  is approximately spherical. 
     Similarly to the pigment  310 , the color toner  300  containing the pigment  310  is also spherical. Thus, when the color toner  300  is placed on the flat surface  500 , a lateral dimension A 2  and a front-rear dimension B 2  of the color toner  300 , viewed from the top in  FIG. 11A , are equal to the lateral dimension A 2  and a vertical dimension C 2 , viewed from the side in  FIG. 11B  of the color toner  300 . 
     The volumetric average particle diameter of the pigment  310  falls within the range of approximately 50 nm to 150 nm. The volumetric average particle diameter of the color toner  300  falls within the range of 3 μm to 9 μm. When the volumetric average particle diameter exceeds 9 μm, the image may have a low resolution. On the other hand, when the volumetric average particle diameter falls below 3 μm, the toner may be charged insufficiently and the developed image may have low quality. 
     Here, in the present exemplary embodiment, a toner having a specific gravity of 1.1 g/cm 3  and a volumetric average particle diameter of 4.7 μm is used as each of the Y toner, the M toner, and the C toner. A toner having a specific gravity of 1.2 g/cm 3  and a volumetric average particle diameter of 4.7 μm is used as the K toner. Thus, the Y toner, the M toner, and the C toner have a mass of 0.6×10 −10  g, and the K toner has a mass of 0.65×10 −10  g. 
     The color toner  300  may contain a compound formed from a divalent or polyvalent metallic element. The compound is added as, for example, a coagulant to form the color toner  300  by emulsion polymerization aggregation. The content of the compound in the color toner  300  falls within a range of, for example, 0.05 percent by mass to 2 percent by mass. 
     Transfer Belt  50   
     As illustrated in  FIG. 4 , the transfer belt  50  is endless and wound around multiple rollers  32 . The transfer belt  50  is in a position of an inverted obtuse triangle, long in the apparatus width direction in a front view. In the present exemplary embodiment, the transfer belt  50  is made of a material obtained by dispersing carbon in polyimide. The transfer belt  50  has a volume resistivity of 12.5 log ohm-cm. 
     Transfer Portion  14   
     The transfer portion  14  includes multiple rollers  32 , around which the transfer belt  50  is wound, and first transfer rollers  52  for various colors, which transfer the toner layers formed on the image carriers  40  for the various colors to the transfer belt  50 . The transfer portion  14  also includes a second transfer portion  54 , which transfers a toner image transferred to the transfer belt  50  to the sheet member P, and eccentric cams  72  (see  FIGS. 1A and 1B ), which move a roller  32 B, described below. 
     Rollers  32   
     The multiple rollers  32  include a roller  32 D disposed on a first end (on the right side) in the apparatus width direction. The roller  32 D rotates the transfer belt  50  in the direction of arrow A (counterclockwise in the drawing) with a rotational force transmitted from a motor, not illustrated. In the present exemplary embodiment, the roller  32 D is a cylindrical metal roller having an outer diameter of 28 mm. 
     The multiple rollers  32  include a roller  32 B, around which the lower end vertex of the transfer belt  50  taking an obtuse triangle position is wound to form an obtuse angle. As illustrated in  FIG. 1A , the roller  32 B includes a rotation shaft  36 , which has a smaller diameter than the outer diameter of the roller  32 B. The roller  32 B also includes a guiderail, not illustrated, for guiding the rotation shaft  36  so that the roller  32 B moves toward and away from the second transfer portion  54 . 
     The roller  32 B faces the second transfer portion  54  with the transfer belt  50  interposed therebetween. A transfer current is fed to the roller  32 B. In the present exemplary embodiment, the roller  32 B is an elastic roller having an outer diameter of 28 mm. The roller  32 B has a surface resistance of 7.3 log ohm/sq. The roller  32 B has a surface hardness of 53 degrees in Asker C hardness. 
     As illustrated in  FIG. 4 , the multiple rollers  32  include a roller  32 T on the upstream side of and adjacent to the roller  32 B in the direction in which the transfer belt  50  rotates (hereinafter referred to as “a belt rotation direction”). The roller  32 T applies a tension to the transfer belt  50 . Specifically, a slop portion of the transfer belt  50  is wound around the roller  32 T. The slop portion of the transfer belt  50  tilts from the horizontal direction. In the present exemplary embodiment, the roller  32 T is a cylindrical metal roller having an outer diameter of 28 mm. 
     Eccentric Cams  72   
     As illustrated in  FIG. 1A , a pair of eccentric cams  72  are disposed so as to hold the roller  32 B therebetween in the apparatus depth direction such that the outer circumferential surfaces of the eccentric cams  72  touch the rotation shaft  36  of the roller  32 B. On the outer circumferential surface of each eccentric cam  72 , an urging member, not illustrated, is disposed. The urging member urges the rotation shaft  36  of the roller  32 B to bring the rotation shaft  36  into contact with the corresponding eccentric cam  72 . 
     In this structure, the eccentric cams  72  rotate with the rotational force of a stepping motor  74  (“the motor  74 ”, below), which rotates the eccentric cams  72 , to move the roller  32 B toward and away from the second transfer portion  54  (see  FIGS. 1A and 2A ). 
     First Transfer Roller  52   
     As illustrated in  FIG. 4 , the first transfer rollers  52  are disposed so as to face the image carriers  40  of the respective colors with the transfer belt  50  interposed therebetween. In the present exemplary embodiment, the first transfer rollers  52  are elastic rollers having an outer diameter of 28 mm. The first transfer rollers  52  have a resistance of 7.71 log ohm and the first transfer rollers  52  have a surface hardness of 30 degrees in Asker C hardness. 
     In this structure, when a transfer current is fed to each of the first transfer rollers  52  of the corresponding color, a transfer electric field is formed between the first transfer roller  52  and the corresponding image carrier  40 . This transfer electric field transfers the toner layers on the image carriers  40  to the transfer belt  50 , so that the transfer belt  50  holds a toner image formed from one or more toner layers. 
     Second Transfer Portion  54   
     As illustrated in  FIG. 1A , the second transfer portion  54  includes an endless elastic belt  64 , and rollers  66  and  68 , around which the elastic belt  64  is wound. 
     In the present exemplary embodiment, the elastic belt  64  is a rubber belt having a thickness of 450 μm and a perimeter of 40 mm. The elastic belt  64  has a volume resistance of 9.2 log ohm. 
     The roller  66  is grounded and disposed so as to hold the transfer belt  50  and the elastic belt  64  between the roller  66  and the roller  32 B. In the present exemplary embodiment, the roller  66  is an elastic roller having an outer diameter of 28 mm. The roller  66  has a resistance of 6.3 log ohm. 
     The roller  68  is located on the downstream side of the roller  66  in the direction in which the sheet member P is transported along the transport path  16  (hereinafter referred to as “a sheet transport direction”). In the present exemplary embodiment, the roller  68  is a cylindrical metal roller having an outer diameter of 20 mm. 
     In this structure, a sheet member P transported while being held between the transfer belt  50  and the second transfer portion  54  is pressed against the transfer belt  50 . When a transfer current is fed to the roller  32 B, a transfer electric field is formed between the roller  32 B and the roller  66  of the second transfer portion  54 . This transfer electric field transfers the toner image on the transfer belt  50  to the sheet member P that is being transported. 
     Setting Portion  58   
     As illustrated in  FIG. 1A , the setting portion  58  drives the motor  74  to rotate the eccentric cams  72 . The setting portion  58  moves the roller  32 B to a position at which the center of rotation of the roller  32 B and the center of rotation of the roller  66  are closest to each other (see  FIGS. 1A and 1B ), and to a position at which the center of rotation of the roller  32 B and the center of rotation of the roller  66  are farthest from each other (see  FIGS. 2A and 2B ). 
     In the present exemplary embodiment, when the roller  32 B is moved to the position at which the center of rotation of the roller  32 B and the center of rotation of the roller  66  are closest to each other, the transfer belt  50  depresses the elastic belt  64  of the second transfer portion  54  (see  FIG. 1B ) by 0.3 mm. In other words, the amount of depression is 0.3 mm. 
     On the other hand, when the roller  32 B is moved to the position at which the center of rotation of the roller  32 B and the center of rotation of the roller  66  are farthest from each other, the transfer belt  50  is spaced 0.5 mm apart from the elastic belt  64  of the second transfer portion  54  (see  FIG. 2B ). In other words, the amount of depression is −0.5 mm. 
     In this structure, the setting portion  58  receives from a controller, not illustrated, information of the mass of toner (toner particles) of the uppermost toner layer in the toner image on the transfer belt  50 . When the mass of the toner is equal to or exceeds a threshold, the setting portion  58  changes the amount of depression by which the transfer belt  50  depresses the elastic belt  64  to −0.5 mm. 
     In the present exemplary embodiment, the threshold of the mass of the toner (toner particles) is set at, for example, 0.2×10 −9  g. 
     When the mass of the toner of the uppermost toner layer in the toner image on the transfer belt  50  falls below the threshold, the setting portion  58  changes the amount of depression by which the transfer belt  50  depresses the elastic belt  64  to 0.3 mm. Thus, the sheet member P to which the toner image is to be transferred is pressed against the transfer belt  50  with a greater pressing force (hereinafter referred to as “a first pressing force”) than in the case where the amount of depression by which the transfer belt  50  depresses the elastic belt  64  is −0.5 mm. 
     In other words, the setting portion  58  presses the sheet member P against the transfer belt  50  with a pressing force smaller than the first pressing force (hereinafter the smaller force is referred to as “a second pressing force”) when the mass of the toner of the uppermost toner layer on the transfer belt  50  is equal to or exceeds the threshold. 
     Evaluations 
     Now, evaluations of an image forming apparatus  910  according to a comparative example and the image forming apparatus  10  according to the present exemplary embodiment are described. Firstly, the structure of the image forming apparatus  910  according to a comparative example is described. Then, the evaluations of the image forming apparatus  910  according to a comparative example and the image forming apparatus  10  according to the present exemplary embodiment are described. 
     Image Forming Apparatus  910  According to Comparative Example 
     Firstly, portions of the image forming apparatus  910  according to a comparative example that differ from those of the image forming apparatus  10  are mostly described. 
     As illustrated in  FIG. 17 , a roller  32 B of the image forming apparatus  910  according to a comparative example is rendered unmovable. The relative positional relationship between the roller  32 B and the roller  66  of the image forming apparatus  910  is the same as that when the center of rotation of the roller  32 B and the center of rotation of the roller  66  in the image forming apparatus  10  are closest to each other. Specifically, the amount of depression by which the transfer belt  50  depresses the elastic belt  64  is maintained constant at 0.3 mm. Thus, the sheet member P to which the toner image is to be transferred is constantly pressed against the transfer belt  50  with the first pressing force, which is greater than the second pressing force. 
     Evaluations 
     The evaluations of the image forming apparatus  10  and the image forming apparatus  910  are described now. 
     Evaluation Specifications 
     Evaluations are performed using a machine obtained by converting Color 1000 Press from Fuji Xerox Co., Ltd. into the image forming apparatus  10  and a machine obtained by converting Color 1000 Press from Fuji Xerox Co., Ltd. into the image forming apparatus  910 . The process speed of the image forming apparatus  10  and the image forming apparatus  910  is set at 524 mm/s. 
     The evaluations are performed in the surrounding of the room temperature of 28° C. and the humidity of 85% RH. 
     The toner mass per area (TMA, mass of toner per unit area) of the Y toner, the M toner, and the C toner is set at 3.3 g/m 2 , the TMA of the K toner is set at 3.7 g/m 2 , and the TMA of the W toner is set at 8.2 g/m 2 . 
     The toner layer forming portion  30 W used in the evaluations is the one disposed downstream (to the left in  FIG. 4 ), in the belt rotation direction, of the non-white color toner layer forming portions  30 Y,  30 M,  30 C, and  30 K, and not the one disposed upstream of the non-white color toner layer forming portions  30 Y,  30 M,  30 C, and  30 K. 
     The evaluations are performed using metallic sheets from Gojo Paper MFG. Co., Ltd. (product No. 215-256, basis weight of 256 g/m 2 , and thickness of 0.3 mm) and metallic sheets from Gojo Paper MFG. Co., Ltd. (product No. 220-1, basis weight of 350 g/m2, and thickness of 0.5 mm). In the following description, the metallic sheets of the basis weight of 256 g/m 2  may be referred to as “ordinary paper sheets”, and the metallic sheets of the basis weight of 350 g/m 2  may be referred to as “thick paper sheets”. 
     Evaluation Images 
     A belt-like solid image (portion C in  FIG. 16  having a width of 10 mm in the sheet transport direction (process direction) for each color is output on each of the ordinary paper sheets and the thick paper sheets from the position 20 mm apart from the leading end of each sheet. 
     Evaluation Method 
     Output images are each visually inspected, and rated “poor” if the image has low image quality due to, for example, toner scattering, or rated “fair” if the image is acceptable as a product even with toner scattering. 
     Evaluation Results 
     Firstly, the evaluation results of the image forming apparatus  910  are described with the table shown in  FIG. 12B . 
     As shown in the table in  FIG. 12B , a toner image is formed with the W toner by the image forming apparatus  910  and, rated “poor” in an evaluation result when using a thick paper sheet. Other toner images are rated “fair” in different evaluation results. As illustrated in  FIG. 16 , the output image rated “poor” in the evaluation result has a portion (portion D in  FIG. 16 ) to which scattering toner adheres at a position apart from the trailing end of the belt-like solid image (portion C in  FIG. 16 ) having a width of 10 mm. 
     The reason why the image is rated “poor” in the evaluation result is considered below. 
     As illustrated in  FIG. 18A , a sheet member P (thick paper sheet) on which a toner image is formed is transported toward the pressing portion (nip portion) formed between the roller  32 B and the second transfer portion  54 . As illustrated in  FIGS. 18B and 19A , when the leading end of the transported sheet member P hits the pressing portion or a portion narrowed between the transfer belt  50  and the elastic belt  64 , the leading end portion of the sheet member P is bent and collides with the transfer belt  50  (portion E in  FIG. 19A ). This collision vibrates the transfer belt  50  and scatters part of the W toner on the transfer belt  50 , and the scattering toner adheres to the rotating transfer belt  50  again. The above-described output image (see  FIG. 16 ) is possibly formed in this manner. As illustrated in  FIG. 19B , the sheet member P having its leading end portion temporarily bent is transported while being held between the transfer belt  50  and the second transfer portion  54  and while being pressed against the transfer belt  50 . 
     Now, the reason why only the W toner scatters is considered. 
       FIG. 13  illustrates a force exerted on each toner when the transfer belt  50  vibrates. The same vibration (acceleration) exerts a larger force on the W toner having a larger mass than a force on the Y, M, C, and K toners having a smaller mass. This is probably the reason why part of the W toner on the transfer belt  50  scatters while the Y, M, C, and K toners on the transfer belt  50  do not scatter. 
     Now, the reason why the W toner do not scatter when transferred to an ordinary paper sheet but scatters when transferred to a thick paper sheet is considered. 
     The thick paper sheet has a larger basis weight than the ordinary paper sheet. In other words, the thick paper sheet has higher flexural rigidity than the ordinary paper sheet. As illustrated in  FIGS. 18B and 19A , the force exerted when the bent leading end portion of the sheet member P collides with the transfer belt  50  is higher in the case where the sheet member P is a thick paper sheet than that in the case where the sheet member P is an ordinary paper sheet. 
     Thus, the acceleration generated in the transfer belt  50  when the thick paper sheet collides with the transfer belt  50  is faster than the acceleration generated in the transfer belt  50  when the ordinary paper sheet collides with the transfer belt  50 . This is possibly the reason why the W toner scatters when the W toner is transferred to the thick paper sheet. 
     The evaluation results of the image forming apparatus  10  are described using the table shown in  FIG. 12A . 
     As described above, the setting portion  58  of the image forming apparatus  10  separates the roller  32 B and the roller  66  from each other when the mass of the toner of the uppermost toner layer in the toner image on the transfer belt  50  is equal to or exceeds the threshold. In the present exemplary embodiment, the threshold of the mass of the toner is 0.20×10 −9  g. As described above, the mass of the W toner (toner particles) is 0.51×10 −9  g, which is greater than the threshold. 
     When the setting portion  58  separates the roller  32 B and the roller  66  from each other when the W toner is used in the uppermost toner layer of the transfer belt  50 , the amount of depression by which the transfer belt  50  depresses the elastic belt  64  is set at −0.5 mm. In other words, when the W toner is to be used, the image forming apparatus  10  sets the pressing force exerted to press the sheet member P against the transfer belt  50  at the second pressing force, which is smaller than the first pressing force. 
     As shown in the table in  FIG. 12A , when the image forming apparatus  10  forms toner images also when using the W toner on the thick paper sheet, all the toner images are rated “fair” in the evaluation results. 
     Now, the reason why the toner image formed on the thick paper sheet with the W toner is rated “fair” in the evaluation result is considered. The relevant toner image formed by the image forming apparatus  910  is rated “poor” in the evaluation result. 
       FIG. 14  shows a graph of the bending of the leading end portion of the sheet member P generated when the thick paper sheet is brought into contact with the pressing portion formed between the roller  32 B and the second transfer portion  54 . The amount of the bending is examined by visually inspecting a captured image of the transported thick paper sheet. 
     Specifically, the bending produced at the leading end portion of the sheet member P when the amount of depression by which the transfer belt  50  depresses the elastic belt  64  is changed to 0.3 mm, 0 mm, −0.3 mm, and −0.5 mm is evaluated. As is clear from the table shown in  FIG. 14 , the bending is reduced as the amount of depression is reduced. In other words, the bending is reduced as the pressing force exerted to press the sheet member P against the transfer belt  50  is reduced. 
     As described above, when the W toner is to be used, the image forming apparatus  10  sets the pressing force exerted to press the sheet member P against the transfer belt  50  at the second pressing force, which is smaller than the first pressing force. Thus, in the image forming apparatus  10 , the bending of the leading end portion of the sheet member P is reduced compared to the case where the image forming apparatus  910  is used. This structure is thus considered to reduce the scattering of the W toner on the transfer belt  50  as a result of the reduction of the force with which the bent leading end portion of the sheet member P collides with the transfer belt  50 . 
     Alternatively, the scattering of the W toner on the transfer belt  50  may be reduced by constantly keeping the pressing force exerted to press the sheet member P against the transfer belt  50  low. 
     The graph in  FIG. 15  shows the relationship between the amount of depression by which the transfer belt  50  depresses the elastic belt  64  and the discharge current produced between the roller  32 B and the roller  66 . Specifically, the vertical axis in the graph shows the discharge current and the horizontal axis in the graph shows the amount of depression. As shown from the graph, the discharge current increases as the amount of depression decreases (as the roller  32 B and the roller  66  are spaced further apart from each other). In other words, the discharge current increases as the pressing force exerted to press the sheet member P against the transfer belt  50  decreases. 
     Thus, when the pressing force exerted to press the sheet member P against the transfer belt  50  is constantly kept small, the discharge current is constantly kept high, so that the durability of the roller  32 B and the roller  66  decreases. Specifically, in the image forming apparatus  10 , the pressing force exerted to press the sheet member P against the transfer belt  50  is reduced only when needed, to minimize the quality degradation of an output image and minimize the durability reduction of the roller  32 B and the roller  66 . 
     Operations of Related Components 
     The operations of related components are described now. 
     First, the case where a toner image is formed by using only the Y, M, C, and K color toners  300  is described. Here, a thick paper sheet is used as the sheet member P. The image forming apparatus  10  that has not started an image forming operation (before job execution) has a setting of the amount of depression by which the transfer belt  50  depresses the elastic belt  64  at 0.3 mm. In other words, the image forming apparatus  10  has a setting of the pressing force exerted to press the sheet member P against the transfer belt  50  at the first pressing force. 
     Toner layers formed by the toner layer forming portions  30 Y,  30 M,  30 C, and  30 K are first-transferred to the rotating transfer belt  50  by the first transfer rollers  52  ( FIG. 4 ). As illustrated in  FIG. 6B , the toner image obtained by superposing the Y toner layer, the M toner layer, the C toner layer, and the K toner layer in this order is formed (held) on the transfer belt  50 . Here, the uppermost one of the toner layers disposed on the transfer belt  50  and constituting the toner image is the K toner layer. The mass of the toner of the uppermost toner layer is 0.7×10 −10  g, which is below the threshold. Thus, the setting portion  58  keeps the amount of depression by which the transfer belt  50  depresses the elastic belt  64  at 0.3 mm. In other words, the setting portion  58  maintains the pressing force exerted to press the sheet member P against the transfer belt  50  at the first pressing force, which is greater than the second pressing force. 
     When the transported sheet member P is then transported while being held between the transfer belt  50  and the second transfer portion  54  and while being pressed against the rotating transfer belt  50 , the superposed toner image on the transfer belt  50  is transferred to the sheet member P (see  FIG. 4 ). 
     Now, described is a case where a toner image including the W toner layer as a base coat for the Y, M, C, and K color toners  300  is formed to enhance the color reproducibility. A device disposed downstream (to the left in  FIG. 4 ) of the non-white color toner layer forming portions  30 Y,  30 M,  30 C, and  30 K in the belt rotation direction is used as the toner layer forming portion  30 W that forms the W toner layer. Thick paper sheets are used as the sheet members P. The image forming apparatus  10  that has not started an image forming operation (before job execution) has a setting of the amount of depression by which the transfer belt  50  depresses the elastic belt  64  at 0.3 mm. In other words, the image forming apparatus  10  has a setting of the pressing force exerted to press the sheet member P against the transfer belt  50  at the first pressing force. 
     The toner layers formed by the toner layer forming portions  30 Y,  30 M,  30 C,  30 K, and  30 W are first-transferred to the rotating transfer belt  50  by the respective first transfer rollers  52  ( FIG. 4 ). As illustrated in  FIG. 6A , a toner image obtained by superposing the Y toner layer, the M toner layer, the C toner layer, the K toner layer, and the W toner layer in this order is formed (held) on the transfer belt  50 . Here, the uppermost one of the toner layers disposed on the transfer belt  50  and constituting the toner image is the W toner layer. The mass information of the W toner is input in advance. The mass of the toner of the uppermost toner layer is 0.51×10 −6  g, which exceeds the threshold. Thus, the setting portion  58  changes the amount of depression by which the transfer belt  50  depresses the elastic belt  64  to −0.5 mm. In other words, the setting portion  58  sets the pressing force exerted to press the sheet member P against the transfer belt  50  at the second pressing force, which is smaller than the first pressing force. 
     The transported sheet member P is transported while being held between the transfer belt  50  and the second transfer portion  54  and while being pressed against the rotating transfer belt  50 , so that the superposed toner image on the transfer belt  50  is transferred to the sheet member P (see  FIG. 4 ). 
     CONCLUSION 
     As described above, when the uppermost one of the toner layers on the transfer belt  50  is the W toner layer, the mass of the toner of the uppermost toner layer is equal to or exceeds the threshold. Thus, the image forming apparatus  10  sets the pressing force exerted to press the sheet member P against the transfer belt  50  at the second pressing force, which is smaller than the first pressing force. This structure prevents the transfer belt  50  from vibrating as a result of the leading end of the transported sheet member P coming into contact with the pressing portion, and thus prevents the W toner on the transfer belt  50  from scattering. 
     In other words, compared to the structure where the pressing force exerted to press the sheet member P against the transfer belt  50  is kept constant at the first pressing force, this structure reduces the amount of the W toner on the transfer belt  50  that scatters due to the impact occurring when the transported sheet member P comes into contact with the pressing portion. 
     Reducing the scattering of the W toner on the transfer belt  50  reduces the quality degradation of the images transferred to the sheet member P compared to the structure where the pressing force exerted to press the sheet member P against the transfer belt  50  is kept constant at the first pressing force. 
     As described above, the image forming apparatus  10  reduces the pressing force exerted to press the sheet member P against the transfer belt  50  only when needed, to minimize the quality degradation of an output image and minimize the durability reduction of the roller  32 B and the roller  66 . 
     Second Embodiment 
     Examples of a transfer device and an image forming apparatus according to a second exemplary embodiment of the present invention are described with reference to  FIGS. 20 to 24B . Here, portions of the second exemplary embodiment that differ from those of the first exemplary embodiment are mostly described. 
     As illustrated in  FIG. 20 , an image forming apparatus  410  according to the second exemplary embodiment includes toner layer forming portions  30  for five colors of yellow (Y), magenta (M), cyan (C), black (K), and silver (V). Two toner layer forming portions  30  are provided for silver (V). The toner layer forming portions  30  for silver (V), yellow (Y), magenta (M), cyan (C), black (K), and silver (V) are arranged side by side in the horizontal direction in this order from the upstream side in the rotation direction of the transfer belt  50  (see arrow A in  FIG. 20 ). 
     Silver Toner  100   
     A silver toner  100  (hereinafter may be referred to as “V toner”) is used in a developing device  46 V for the toner layer forming portion  30 V. 
     As illustrated in  FIG. 22 , the silver toner  100  (flat toner) contains a flat pigment  110  and a binding resin  120 . The flat pigment  110  is formed from aluminum (an example of a metal). A known resin material is used as the binding resin  120 , and the binding resin  120  has lower electric conductivity than the flat pigment  110 . 
     As illustrated in  FIG. 23B , when the flat pigment  110  is placed on the flat surface  500  and viewed from the side, the flat pigment  110  has a dimension X 3  in the lateral direction that is longer than a dimension Y 3  in the vertical direction. 
     When the flat pigment  110  illustrated in  FIG. 23B  is viewed from the top, the flat pigment  110  spreads widely as illustrated in  FIG. 23A  unlike when viewed from the side. The flat pigment  110  has a pair of reflection surfaces  110 A facing upward and downward when the flat pigment  110  is placed on the flat surface  500  (see  FIG. 23B ). As described above, the flat pigment  110  has a flat shape. 
     Since the flat pigment  110  has a flat shape, the silver toner  100  containing the flat pigment  110  also has a flat shape, following the contour of the flat pigment  110 . Thus, when the silver toner  100  is placed on the flat surface  500  and viewed from the side, the silver toner  100  has a dimension A 3  in the lateral direction longer than the dimension C 3  in the vertical direction, as illustrated in  FIG. 24B . 
     When the silver toner  100  illustrated in  FIG. 24B  is viewed from the top, the silver toner  100  spreads widely to have a substantially circular shape (substantially elliptic shape) as illustrated in  FIG. 24A , unlike when viewed from the side. 
     Here, the relationship A 3 ≥B 3 &gt;C 3  holds true, where A 3  denotes the maximum length (maximum diameter) of the silver toner  100  viewed from the top, B 3  denotes an orthogonal length orthogonal to the maximum length A 3 , and C 3  denotes a thickness of the silver toner  100  viewed from the side (dimension in the vertical direction). 
     In the present exemplary embodiment, an example used as the V toner has a specific gravity of 1.6 g/cm 3 , a maximum length A 3  of 12 μm, an orthogonal length B 3  of 12 μm, and a thickness C 3  of 2 μm. Thus, the V toner (toner particles) has a mass of 0.24×10 −9  g. 
     The maximum length A 3 , the orthogonal length B 3 , and the thickness C 3  are obtained by observing the toner in an enlarged manner using a color laser microscope “VK-9700” (from KEYENCE CORPORATION) and by calculating the maximum length of the toner flat surface using image processing software. 
     The silver toner  100  is used as a base coat for the non-white colors on the sheet member P. Specifically, the solid layer (solid image) of the silver toner  100  is formed on the sheet member P as a base coat for the non-white colors to provide glossiness to the toner image. 
     Described is a case where this structure forms a toner image including a V toner layer for use as a base coat for the Y, M, C, and K color toners  300  to enhance the image glossiness. A device disposed on the downstream side (to the left in  FIG. 20 ) of the non-white color toner layer forming portions  30 Y,  30 M,  30 C, and  30 K in the belt rotation direction is used as the toner layer forming portion  30 V that forms the V toner layer. A thick paper sheet is used as the sheet member P. 
     Toner layers formed by the toner layer forming portions  30 Y,  30 M,  30 C,  30 K, and  30 V are first-transferred to the rotating transfer belt  50  by the first transfer rollers  52  ( FIG. 20 ). As illustrated in  FIG. 21 , a toner image obtained by superposing the Y toner layer, the M toner layer, the C toner layer, the K toner layer, and the V toner layer in this order is formed (held) on the transfer belt  50 . Here, the uppermost one of the toner layers disposed on the transfer belt  50  and constituting the toner images is the V toner layer. The mass of the toner of the uppermost toner layer is 0.24×10 −9  g, which exceeds the threshold. Thus, the setting portion  58  changes the amount of depression by which the transfer belt  50  depresses the elastic belt  64  to −0.5 mm. In other words, the setting portion  58  sets the pressing force exerted to press the sheet member P against the transfer belt  50  at the second pressing force, which is smaller than the first pressing force. 
     The transported sheet member P is then transported while being held between the transfer belt  50  and the second transfer portion  54  and while being pressed against the rotating transfer belt  50 . Thus, the superposed toner image on the transfer belt  50  is transferred to the sheet member P (see  FIG. 20 ). 
     As described above, when the uppermost one of the toner layers on the transfer belt  50  is the V toner layer, the mass of the toner of the uppermost toner layer exceeds the threshold. Thus, the image forming apparatus  10  sets the pressing force exerted to press the sheet member P against the transfer belt  50  at the second pressing force, which is smaller than the first pressing force. This structure reduces the amount of the V toner on the transfer belt  50  that scatters in response to the impact exerted when the sheet member P comes into contact with the pressing portion, compared to the structure in which the pressing force exerted to press the sheet member P against the transfer belt  50  is kept constant at the first pressing force. 
     Other operations are the same as those in the case of the first exemplary embodiment. 
     Third Embodiment 
     Examples of a transfer device and an image forming apparatus according to a third exemplary embodiment of the present invention are described with reference to  FIGS. 25A and 25B . Portions of the third exemplary embodiment that differ from those of the first exemplary embodiment are mostly described. 
     As illustrated in  FIGS. 25A and 25B , an image forming apparatus  510  according to the third exemplary embodiment includes a setting portion  558 , which drives the motor  74  to rotate the eccentric cams  72 . The setting portion  558  changes the amount of depression by which the transfer belt  50  depresses the elastic belt  64  to −0.5 mm, when the mass of the toner (toner particles) of the uppermost toner layer in the toner images on the transfer belt  50  is equal to or exceeds the threshold and the basis weight of the sheet member P is equal to or exceeds the threshold. In other words, the setting portion  558  keeps the pressing force exerted to press the thick paper sheet against the transfer belt  50  at the first pressing force, which is greater than the second pressing force. 
     Here, in the present exemplary embodiment, the threshold of the basis weight of the sheet member P is 350 g/m 2 . 
     In this structure, when an ordinary paper sheet is used as the sheet member P, the setting portion  558  keeps the amount of depression by which the transfer belt  50  depresses the elastic belt  64  at 0.3 mm even when the uppermost one of the toner layers on the transfer belt  50  is the W toner layer. In other words, the setting portion  558  keeps the pressing force exerted to press the ordinary paper sheet against the transfer belt  50  at the first pressing force. As is clear from the table in  FIG. 12B , when the ordinary paper sheet is used, the toner image is rated “fair” in the evaluation result even when the amount of depression is 0.3 mm (when the pressing force is set at the first pressing force). 
     In this manner, when an ordinary paper sheet is used, the amount of depression by which the transfer belt  50  depresses the elastic belt  64  is set at 0.3 mm even when the uppermost one of the toner layers on the transfer belt  50  is the W toner layer. In this case, the amount of electric discharge is reduced compared to the case where the amount of depression is −0.5 mm. This structure reduces the durability reduction of the roller  32 B and the roller  66 . 
     Other operations are the same as those of the first exemplary embodiment. 
     Fourth Exemplary Embodiment 
     Examples of a transfer device and an image forming apparatus according to a fourth exemplary embodiment of the present invention are described with reference to  FIGS. 26A, 26B, and 27 . Portions of the fourth exemplary embodiment that differ from those of the first exemplary embodiment are mostly described. 
     As illustrated in  FIGS. 26A and 26B , an image forming apparatus  610  according to the fourth exemplary embodiment includes a setting portion  658 , which drives the motor  74  to rotate the eccentric cams  72 . The setting portion  658  sets the pressing force exerted to press the sheet member P against the transfer belt  50  at the second pressing force, which is smaller than the first pressing force, and reduces the transportation speed of the sheet member P, when the mass of the toner of the uppermost toner layer on the transfer belt  50  is equal to or exceeds the threshold. 
     Specifically, a transport device  618  (see  FIG. 27 ), which is an example of a transporting device that transports the sheet members P, transports the sheet members P at a first transportation speed or a second transportation speed, which is slower than the first transportation speed. The first transportation speed is a speed at which the transport device  18  according to the first exemplary embodiment transports the sheet members P. 
     The toner layer forming portions  30  and a transfer portion  614 , serving as an example of a transfer body, are operable at a first process speed, at which toner images are transferred to the sheet member P transported at the first transportation speed, and a second process speed, at which toner images are transferred to the sheet member P transported at the second transportation speed. The first process speed is a speed at which the transfer portion  14  according to the first exemplary embodiment transfers the toner images to the sheet member P. 
     In this structure, the setting portion  658  sets the pressing force exerted to press the sheet member P against the transfer belt  50  at the second pressing force, when the mass of the toner of the uppermost toner layer on the transfer belt  50  is equal to or exceeds the threshold. The setting portion  658  causes the transport device  618  to transport the sheet member P at the second transportation speed, and causes the toner layer forming portions  30  and the transfer portion  614  to transfer the toner images to the sheet member P at the second process speed. 
     In this manner, the transportation speed of the sheet member P is reduced when the image quality is more likely to be reduced by scattering of the W toner. Thus, bending of the leading end portion of the sheet member P as a result of the leading end of the sheet member P coming into contact with the pressing portion is reduced compared to the case where the transportation speed is kept at the constant rate. 
     Thus, reducing the transportation speed of the sheet member P reduces the quality degradation of the image transferred to the sheet member P compared to the case where the transportation speed is kept at the constant rate. 
     Other operations are the same as those of the first exemplary embodiment. 
     Fifth Exemplary Embodiment 
     An example of an image forming apparatus according to a fifth exemplary embodiment of the present invention is described with reference to  FIGS. 28A, 28B, and 29 . Portions of the fifth exemplary embodiment that differ from those of the first exemplary embodiment are mostly described. 
     As illustrated in  FIGS. 28A and 28B , an image forming apparatus  710  according to a fifth exemplary embodiment includes a setting portion  758 , which drives the motor  74  to rotate the eccentric cams  72 . The setting portion  758  sets the pressing force exerted to press the sheet member P against the transfer belt  50  at the second pressing force, only while the leading end portion of the sheet member P is passing through the pressing portion, when the mass of the toner of the uppermost toner layer on the transfer belt  50  is equal to or exceeds the threshold. 
     Specifically, a sensor  612 , which detects the leading end of the transported sheet member P, is disposed upstream of the pressing portion formed between the roller  32 B and the second transfer portion  54  in the sheet transport direction. 
     In this structure, the setting portion  758  that has received detection information of the sensor  612  changes the amount of depression by which the transfer belt  50  depresses the elastic belt  64  from 0.3 mm to −0.5 mm only during a predetermined time period. In other words, the setting portion  758  sets the pressing force exerted to press the sheet member P against the transfer belt  50  at the second pressing force, which is smaller than the first pressing force, only during a predetermined time period. Specifically, the amount of depression is changed from 0.3 mm to −0.5 mm only while a portion of the sheet member P within a range of 2 mm to 7 mm from the leading end (in the range F illustrated in  FIG. 29 ) passes through the pressing portion. 
     Specifically, “the leading end portion of the sheet member P” is limited to the range of the sheet member P 2 mm to 7 mm from the leading end. 
     In this manner, the setting portion  758  sets a pressing force exerted to press the sheet member P against the transfer belt  50  as the second pressing force, only while the leading end portion is passing through the pressing portion, when the mass of the toner of the uppermost toner layer on the transfer belt  50  is equal to or exceeds the threshold. 
     Thus, while the transfer belt  50  and the second transfer portion  54  hold the sheet member P therebetween, the reduction of the durability of the roller  32 B and the roller  66  is reduced compared to the case where the sheet member P is continuously pressed with the second pressing force. 
     Although specific exemplary embodiments of the present invention are described in detail, the present invention is not limited to these exemplary embodiment. It is clear to persons having ordinary skill in the art that the present invention may be embodied in various other exemplary embodiments within the scope of the present invention. For example, in the above-described exemplary embodiments, each of the setting portions  58 ,  558 ,  658 , and  758  sets the pressing force exerted to press the sheet member P against the transfer belt  50  at the second pressing force, when the mass of the toner of the uppermost toner layer on the transfer belt  50  is equal to or exceeds the threshold. Alternatively, each of the setting portions  58 ,  558 ,  658 , and  758  may set the pressing force exerted to press the sheet member P against the transfer belt  50  at the second pressing force, when the toner of the uppermost toner layer on the transfer belt  50  contains a pigment formed from metal or a metallic oxide. 
     Specifically, each of the setting portions  58 ,  558 ,  658 , and  758  may set the pressing force exerted to press the sheet member P against the transfer belt  50  at the second pressing force, when the toner of the uppermost one of the toner layers constituting the toner image and disposed on the transfer belt  50  is a W toner or a V toner. This structure reduces the quality degradation of an image transferred to the sheet member P compared to the structure in which the pressing force exerted to press the sheet member P against the transfer belt  50  is kept constant at the first pressing force. 
     In the third exemplary embodiment, the setting portion  558  sets the pressing force exerted to press the sheet member P against the transfer belt  50  at the second pressing force, when the mass of the toner of the uppermost toner layer on the transfer belt  50  is equal to or exceeds the threshold and the basis weight of the sheet member P is equal to or exceeds the threshold. Instead, the setting portion  558  may set the pressing force exerted to press the sheet member P against the transfer belt  50  at the second pressing force, when the toner of the uppermost toner layer on the transfer belt  50  contains a pigment formed from a metal or a metallic oxide and the basis weight of the sheet member P is equal to or exceeds the threshold. 
     In the fourth exemplary embodiment, the setting portion  658  sets the pressing force exerted to press the sheet member P against the transfer belt  50  at the second pressing force, and reduces the transportation speed of the sheet member P, when the mass of the toner of the uppermost toner layer on the transfer belt  50  is equal to or exceeds the threshold. Instead, the setting portion  658  may set the pressing force exerted to press the sheet member P against the transfer belt  50  at the second pressing force, and reduce the transportation speed of the sheet member P, when the toner of the uppermost toner layer on the transfer belt  50  contains a pigment formed from a metal or a metallic oxide. 
     In the fifth exemplary embodiment, the setting portion  758  sets the pressing force exerted to press the sheet member P against the transfer belt  50  at the second pressing force only while the leading end portion of the sheet member P is passing through the pressing portion, when the mass of the toner of the uppermost toner layer on the transfer belt  50  is equal to or exceeds the threshold. Instead, the setting portion  758  may set the pressing force exerted to press the sheet member P against the transfer belt  50  at the second pressing force, only while the leading end portion of the sheet member P is passing through the pressing portion, when the toner of the uppermost toner layer on the transfer belt  50  contains a pigment formed from a metal or a metallic oxide. 
     In the above-described exemplary embodiment, the volumetric average particle diameter is used to calculate the mass of the white toner  200  or the color toners  300 . Instead, the particle diameter averaged by the number of particles may be used to calculate the mass. The particle diameter averaged by the number of particles may be measured by a charge spectrometer (E-Spart ANALYZER) from HOSOKAWA MICRON CORPORATION. This is a measuring device that detects the movement of particles in the aerial vibration field in the electric field by a laser Doppler method and concurrently measures the amount of electric charge and the particle diameter of individual particles from the data. The data of 3000 toner particles are input to this device and the average of the individual particle diameter data is the particle diameter averaged by the number of particles. 
     In each of the above-described exemplary embodiment, the present application is described using a tandem image forming apparatus  10  that develops a latent image on a single image carrier  40  with a single developing device  46 . Instead, the image forming apparatus  10  may be a revolver (5 to 6 cycle) image forming apparatus that develops a latent image on a single image carrier with multiple developing devices. 
     The foregoing description of the exemplary embodiments of the present invention has been provided for the purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise forms disclosed. Obviously, many modifications and variations will be apparent to practitioners skilled in the art. The exemplary embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various exemplary embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.