Patent Publication Number: US-9429879-B2

Title: Image forming apparatus having imaging units using toner with different particle sizes

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2014-166078 filed Aug. 18, 2014. 
     BACKGROUND 
     Technical Field 
     The present invention relates to an image forming apparatus. 
     SUMMARY 
     According to an aspect of the invention, there is provided an image forming apparatus includes four first imaging units that use standard toners for colors of yellow, magenta, cyan, and black, respectively, a second imaging unit that uses a toner having a particle size smaller than a smallest one of particle sizes of the four standard toners, and a transfer unit that transfers first images formed by the respective first imaging units and a second image formed by the second imaging unit to a recording medium such that the first images are provided on the recording medium and the second image is provided on the first images. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Exemplary embodiments of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  illustrates an image forming section of an image forming apparatus according to a first exemplary embodiment of the present invention; 
         FIGS. 2A and 2B  are sectional views of a set of toner images that is output by the image forming apparatus according to the first exemplary embodiment of the present invention; 
         FIGS. 3A and 3B  are sectional views of a set of toner images that is output by an image forming apparatus according to an embodiment that is comparative to the first exemplary embodiment of the present invention; 
         FIG. 4  is a table that summarizes the results of evaluation of images that are output by the image forming apparatus according to the first exemplary embodiment of the present invention and by the image forming apparatus according to the comparative embodiment; 
         FIG. 5  illustrates one of toner-image-forming units included in the image forming apparatus according to the first exemplary embodiment of the present invention; 
         FIG. 6  illustrates the image forming apparatus according to the first exemplary embodiment of the present invention; 
         FIG. 7  illustrates an image forming section of an image forming apparatus according to a second exemplary embodiment of the present invention; 
         FIG. 8  illustrates an image forming section of an image forming apparatus according to a third exemplary embodiment of the present invention; and 
         FIGS. 9A and 9B  are sectional views of a set of toner images that is output by the image forming apparatus according to the third exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     First Exemplary Embodiment 
     An image forming apparatus according to a first exemplary embodiment of the present invention will now be described with reference to  FIGS. 1 to 6 . In each of the drawings, arrow H representing the vertical direction corresponds to the height direction of the apparatus, and arrow W representing the horizontal direction corresponds to the width direction of the apparatus. 
     Referring to  FIG. 6 , an image forming apparatus  10  includes an image forming section  12  that forms an image electrophotographically, and plural transporting members (not denoted) that transport, along a transport path  16 , a sheet member P (an exemplary recording medium) on which the image is to be formed. 
     The image forming apparatus  10  further includes a cooling unit  20  that cools the sheet member P on which the image has been formed, a decurling unit  22  that decurls the sheet member P, and an image inspecting unit  24  that inspects the image formed on the sheet member P. 
     The image forming apparatus  10  further includes a reversal path  26  along which the sheet member P having the image on one side thereof is reversed and is transported toward the image forming section  12  again for the formation of another image on the other side of the sheet member P. 
     In the image forming apparatus  10  configured as described above, the image forming section  12  forms an image (a set of toner images) on a sheet member P that is transported along the transport path  16 , and the sheet member P having the image thereon is transported through the cooling unit  20 , the decurling unit  22 , and the image inspecting unit  24  in that order and is discharged to the outside of the image forming apparatus  10 . 
     In a case where another image is to be formed on the other side of the sheet member P, the sheet member P already having the image on one side thereof is transported along the reversal path  26 , and another image is formed on the other side of the sheet member P by the image forming section  12 . 
     Image Forming Section 
     Referring to  FIG. 1 , the image forming section  12  includes plural toner-image-forming units  30  that form toner images in different colors, respectively, and a transfer unit  32  that transfers the toner images formed by the toner-image-forming units  30  to a sheet member P. The image forming section  12  further includes a fixing device  34  that fixes the toner images that have been transferred to the sheet member P by the transfer unit  32  to the sheet member P. 
     The plural toner-image-forming units  30  are provided so that toner images in respectively different colors are formed. In the first exemplary embodiment, five toner-image-forming units  30  for five respective colors of a special color (V), yellow (Y), magenta (M), cyan (C), and black (K) are provided. Suffixes V, Y, M, C, and K provided to reference numerals in  FIG. 1  represent the respective colors. In the first exemplary embodiment, the special color (V) is a transparent color (hereinafter referred to as transparent color (V)) that gives a gloss to the image, whereas yellow (Y), magenta (M), cyan (C), and black (K) are standard colors for outputting a color image. Hereinafter, the suffixes V, Y, M, C, and K provided to reference numerals are omitted if there is no need to distinguish the elements by the colors of the transparent color (V), yellow (Y), magenta (M), cyan (C), and black (K). 
     The toner-image-forming units  30  provided for the respective colors basically have the same configuration except the toners to be used. Referring to  FIG. 5 , the toner-image-forming units  30  each include a cylindrical image carrying member  40  that is rotatable, a charger  42  that charges the image carrying member  40 , an exposure device  44  that emits exposure light toward the image carrying member  40  having been charged and thus forms an electrostatic latent image on the image carrying member  40 , and a developing device  46  that develops the electrostatic latent image into a toner image with a developer G containing a corresponding one of the toners. Details of the developing device  46  will be described separately below. 
     The image carrying member  40  for each of the colors is in contact with a transfer belt  50  (to be described in detail below) that is rotatable. As illustrated in  FIG. 1 , the toner-image-forming units  30  for the transparent color (V), yellow (Y), magenta (M), cyan (C), and black (K) are arranged side by side in the horizontal direction in that order from the upstream side in the direction of rotation of the transfer belt  50  (see the arrow illustrated in  FIG. 1 ). 
     The toner-image-forming units  30 Y,  30 M,  30 C, and  30 K (exemplary first imaging units) form images in yellow (Y), magenta (M), cyan (C), and black (K) (exemplary standard colors) with standard toners T 1  (see  FIG. 2A ) for the colors of yellow (Y), magenta (M), cyan (C), and black (K), respectively. 
     The toner-image-forming unit  30 V (an exemplary second imaging unit) forms an image in the transparent color (V) (an exemplary special color) with a clear toner T 2  (an exemplary special toner). 
     The transfer unit  32  includes the transfer belt  50 , on which the toner images in the respective colors formed on the respective image carrying members  40  are superposed one on top of another and from which the set of toner images is transferred to a sheet member P. Details of the transfer unit  32  will be described separately below. 
     As illustrated in  FIG. 1 , the fixing device  34  includes a fixing belt  60  that is stretched around plural rollers (not denoted) and is to be heated, and a pressure roller  62  that presses the sheet member P against the fixing belt  60 . 
     In such a configuration, the sheet member P to which the set of toner images has been transferred is nipped between the fixing belt  60  that is under rotation and the pressure roller  62 , whereby the set of toner images is fixed to the sheet member P. 
     Featured Configuration 
     The developing devices  46  included in the toner-image-forming units  30 , the particle sizes (volume-mean particle sizes) of the toners used in the developing devices  46 , the transfer unit  32 , and other featured elements will now be described. 
     Developing Devices 
     The developing devices  46  (see  FIG. 5 ) develop the electrostatic latent images on the outer circumferential surfaces of the image carrying members  40  into toner images with the toners T contained in the developers G, respectively. 
     The standard toners T 1  for the standard colors of yellow (Y), magenta (M), cyan (C), and black (K) all have a volume-mean particle size of 5.8 μm. The clear toner T 2  for the transparent color (V) has a volume-mean particle size of 3.8 μm. That is, in the first exemplary embodiment, the standard toners T 1  have a larger volume-mean particle size than the clear toner T 2 . 
     The volume-mean particle sizes of the toners T are measured with a particle-size-distribution-measuring machine COULTER COUNTER (a registered trademark) Multisizer (a trademark) II of Beckman Coulter, Inc., and with an electrolyte ISOTON II of Beckman Coulter, Inc. The measurement is conducted as follows. First, 0.5 mg to 50 mg of a sample is added to a surfactant as a dispersant, for example, 2 ml of a 5% aqueous solution of sodium alkylbenzenesulfonate. The mixture is added to 100 ml to 150 ml of the electrolyte. The electrolyte in which the sample has thus been suspended is dispersed for about one minute with an ultrasonic dispersion machine. Then, the particle-size distribution of the sample is measured with COULTER COUNTER Multisizer II by using an aperture of 100 μm. The number of particles to be measured is 50,000. The particle-size distribution thus measured is graphed in the form of cumulative undersize distribution represented on the basis of predefined particle-size ranges (channels), the values of which become larger from the smallest one. The particle-size range that corresponds to a cumulative percentage of 50% in volume is defined as a volume-mean particle size D50 v, which is taken as the volume-mean particle size of the sample. 
     Transfer Unit 
     As illustrated in  FIG. 1 , the transfer unit  32  includes the transfer belt  50  stretched around the plural rollers (not denoted) and being rotatable in the direction of the arrow illustrated in  FIG. 1 , and first transfer rollers  52  provided across the transfer belt  50  from the respective image carrying members  40 . The first transfer rollers  52  transfer the respective toner images on the image carrying members  40  to the transfer belt  50 . The transfer unit  32  further includes a roller  56  that also stretches the transfer belt  50 , and a second transfer roller  54  provided across the transfer belt  50  from the roller  56  and that transfers the set of toner images on the transfer belt  50  to a sheet member P. 
     In such a configuration, the toner images formed by the respective toner-image-forming units  30  are transferred, for the first transfer, to the transfer belt  50  that is under rotation by the respective first transfer rollers  52 . Subsequently, the toner images thus transferred to the transfer belt  50  for the first transfer are transferred, for the second transfer, to the sheet member P by the second transfer roller  54 . 
     In the first transfer, the toner images in the transparent color (V), yellow (Y), magenta (M), cyan (C), and black (K) are superposed one on top of another in that order on the transfer belt  50 . When the set of toner images is transferred to the sheet member P for the second transfer, the order of the toner images is reversed. That is, in the second transfer, the toner images are superposed on the sheet member P in the order of that in black (K), that in cyan (C), that in magenta (M), that in yellow (Y), and that in the transparent color (V). 
     In other words, standard-color images (exemplary first images) in the standard colors of black (K), cyan (C), magenta (M), and yellow (Y) are superposed in that order on the sheet member P, and a clear image (an exemplary second image) in the transparent color (V) is superposed over the set of standard-color images. 
     Other Features 
     A set of toner images formed on a sheet member P by the image forming apparatus  10  according to the first exemplary embodiment and a set of toner images formed on a sheet member P by an image forming apparatus according to an embodiment comparative to the first exemplary embodiment will now be described with reference to  FIGS. 2A, 2B, 3A , and  3 B. In  FIGS. 2A and 3A , the difference in the size of toner particles forming the toner images is exaggerated for easy recognition. 
     First, a set of toner images formed on a sheet member P by the image forming apparatus according to the comparative embodiment will be described with reference to  FIGS. 3A and 3B . 
     Toners used in the image forming apparatus according to the comparative embodiment are a standard toner T 5  for magenta (M) having a volume-mean particle size of 3.8 μm, and a clear toner T 6  for the transparent color (V) having a volume-mean particle size of 5.8 μm. 
     In the image forming apparatus according to the comparative embodiment, toner images are formed by using the toner-image-forming unit  30 M and the toner-image-forming unit  30 V, respectively, whereby an image having a standard color (magenta) with a gloss is output on a sheet member P. 
       FIG. 3A  is a sectional view illustrating a standard-color image formed on the sheet member P and a clear image formed over the standard-color image, the two images being yet to be fixed to the sheet member P. As illustrated in  FIG. 3A , some particles of the clear toner T 6  having a volume-mean particle size of 5.8 μm push away particles of the standard toner T 5  having a volume-mean particle size of 3.8 μm, and are in contact with the sheet member P. 
       FIG. 3B  is a sectional view illustrating the standard-color image and the clear image that have been fixed to the sheet member P. In the state where the standard-color image and the clear image are fixed to the sheet member P as illustrated in  FIG. 3B , some portions of the clear image are in contact with the sheet member P. Therefore, the resulting magenta (M) image with a gloss that has been output on the sheet member P has nonuniformity. 
     Now, a set of toner images formed on a sheet member P by the image forming apparatus  10  according to the first exemplary embodiment will be described with reference to  FIGS. 2A and 2B . 
     Toners used in the image forming apparatus  10  according to the first exemplary embodiment are a standard toner T 1  for magenta (M) having a volume-mean particle size of 5.8 μm, and a clear toner T 2  for the transparent color (V) having a volume-mean particle size of 3.8 μm. The other specifications of the image forming apparatus  10  are the same as those of the image forming apparatus according to the comparative embodiment. 
       FIG. 2A  is a sectional view illustrating a standard-color image formed on the sheet member P and a clear image formed over the standard-color image, the two images being yet to be fixed to the sheet member P. 
     Since the standard toner T 1  for magenta (M) has a larger volume-mean particle size than the clear toner T 2 , particles of the clear toner T 2  do not push away particles of the standard toner T 1 , as illustrated in  FIG. 2A . Thus, the contact between the particles of the clear toner T 2  and the sheet member P is suppressed. 
       FIG. 2B  is a sectional view illustrating the standard-color image and the clear image that have been fixed to the sheet member P. In the state where the standard-color image and the clear image are fixed to the sheet member P as illustrated in  FIG. 2B , the clear image is out of contact with the sheet member P. Thus, the occurrence of nonuniformity in the output image is suppressed. 
     As described above, image nonuniformity may occur in the set of toner images formed by the image forming apparatus according to the comparative embodiment but is suppressed in the set of toner images formed by the image forming apparatus  10  according to the first exemplary embodiment. 
     Evaluation 
     Image nonuniformity that may occur in working examples and comparative examples of the first exemplary embodiment is evaluated by using Color 1000 Press of Fuji Xerox Co., Ltd. 
     Evaluation Method 
     The evaluation is conducted by using J paper of Fuji Xerox InterField Co., Ltd. having a basis weight of 82 g/m 2 , and coated paper OS Coat W of Fuji Xerox Co., Ltd. as sheet members P. Toner images are formed on five pieces of J paper and on five pieces of coated paper, i.e., on the total of ten sheet members P, for each of different sets of specifications. Then, nonuniformity in the toner images is evaluated. 
     The toner images formed are each a patch image of 20 mm×20 mm. The patch image is composed of a standard-color image with an image coverage of 100% and a clear image with an image coverage of 100%, the clear image being formed over the standard-color image, which has the magenta (M) color. 
     The images thus output are visually examined, and the occurrence of image nonuniformity is evaluated. 
     SPECIFICATIONS OF EXAMPLES 
     Specifications of the examples are as follows (see  FIG. 4 ). 
     1. Working Example 1 
     Volume-mean particle size of standard toner T 1 : 5.8 μm 
     Volume-mean particle size of clear toner T 2 : 3.8 μm 
     2. Working Example 2 
     Volume-mean particle size of standard toner T 1 : 7.5 μm 
     Volume-mean particle size of clear toner T 2 : 5.8 μm 
     3. Comparative Example 1 
     Volume-mean particle size of standard toner T 5 : 3.8 μm 
     Volume-mean particle size of clear toner T 6 : 5.8 μm 
     4. Comparative Example 2 
     Volume-mean particle size of standard toner T 5 : 3.8 μm 
     Volume-mean particle size of clear toner T 6 : 7.5 μm 
     In each of Working Examples 1 and 2, the standard toner T 1  used for the standard-color image formed on the sheet member P has a larger volume-mean particle size than the clear toner T 2  used for the clear image formed over the standard-color image. 
     In each of Comparative Examples 1 and 2, the standard toner T 5  used for the standard-color image formed on the sheet member P has a smaller volume-mean particle size than the clear toner T 6  used for the clear image formed over the standard-color image. 
     The other specifications not listed above are all common to Working Examples 1 and 2 and Comparative Examples 1 and 2. 
     Results 
     If there is no image nonuniformity or if the level of image nonuniformity is acceptable to the user, the result is regarded as “GOOD.” If the level of image nonuniformity is not acceptable to the user, the result is regarded as “NO GOOD.” 
     As summarized in the table illustrated in  FIG. 4 , the results of Working Examples 1 and 2 are “GOOD,” whereas the results of Comparative Examples 1 and 2 are “NO GOOD.” 
     Review 
     In each of Working Examples 1 and 2, the clear toner T 2  has a smaller volume-mean particle size than the standard toner T 1 , and, as described above, particles of the clear toner T 2  do not therefore push away particles of the standard toner T 1 , suppressing the contact between the clear toner T 2  and the sheet member P. Hence, the level of image nonuniformity, if any, is acceptable to the user. 
     In each of Comparative Examples 1 and 2, the clear toner T 6  has a larger volume-mean particle size than the standard toner T 5 , and, as described above, some particles of the clear toner T 6  therefore push away particles of the standard toner T 5  and come into contact with the sheet member P. Hence, the level of image nonuniformity is not acceptable to the user. 
     Summary of Featured Configurations 
     In the first exemplary embodiment, the standard toner T 1  used for the standard-color image formed on the sheet member P has a larger volume-mean particle size than the clear toner T 2  used for the clear image formed over the standard-color image. In other words, the standard toner T 1  having a relatively large volume-mean particle size is provided as a lower layer lying on the sheet member P, and the clear toner T 2  having a relatively small volume-mean particle size is provided as an upper layer lying over the lower layer. As is understood from the results of the above evaluation, such a difference in the particle size of the toners suppresses the occurrence of nonuniformity in the output image. 
     Second Exemplary Embodiment 
     An image forming apparatus according to a second exemplary embodiment of the present invention will now be described with reference to  FIG. 7 . Elements that are the same as those described in the first exemplary embodiment are denoted by corresponding ones of the reference numerals used in the first exemplary embodiment, and description thereof is thus omitted. Differences from the first exemplary embodiment will be discussed basically. 
     In an image forming apparatus  70  according to the second exemplary embodiment, a green color (hereinafter referred to as green (G)) is employed as a special color that widens the gamut of colors that are reproducible in the output image. As illustrated in  FIG. 7 , toner-image-forming units  30  for yellow (Y), magenta (M), cyan (C), black (K), and green (G) are arranged side by side in the horizontal direction and in that order from the upstream side in the direction of rotation of the transfer belt  50  (see the arrow illustrated in  FIG. 7 ). 
     The toner-image-forming units  30 Y,  30 M,  30 C, and  30 K (exemplary first imaging units) form standard-color images in yellow (Y), magenta (M), cyan (C), and black (K), respectively. The toner-image-forming unit  30 G (an exemplary second imaging unit) forms a green (G) image. 
     A green toner T 3  used for forming the green (G) image has a volume-mean particle size of 5.8 The standard toner T 1  used for forming each of the images in the standard colors of yellow (Y), magenta (M), cyan (C), and black (K) has a volume-mean particle size of 3.8 μm. 
     In such a configuration, the toner images in yellow (Y), magenta (M), cyan (C), black (K), and green (G) are superposed one on top of another in that order on the transfer belt  50  in the first transfer. When the set of toner images is transferred to a sheet member P for the second transfer, the order of the toner images thus superposed is reversed. That is, in the second transfer, the toner images are superposed on the sheet member P in the order of that in green (G), that in black (K), that in cyan (C), that in magenta (M), and that in yellow (Y). 
     The green toner T 3  used for the green image formed on the sheet member P has a larger volume-mean particle size than the standard toners T 1  used for the standard-color images formed over the green image. In other words, the green toner T 3  having a relatively large volume-mean particle size is provided as a lower layer lying on the sheet member P, and the standard toners T 1  having a relatively small volume-mean particle size are provided as an upper layer lying over the lower layer. 
     As in the first exemplary embodiment, such a difference in the particle size of the toners suppresses the occurrence of nonuniformity in the output image. 
     Third Exemplary Embodiment 
     An image forming apparatus according to a third exemplary embodiment of the present invention will now be described with reference to  FIGS. 8, 9A, and 9B . Elements that are the same as those described in the first exemplary embodiment are denoted by corresponding ones of the reference numerals used in the first exemplary embodiment, and description thereof is thus omitted. Differences from the first exemplary embodiment will be discussed basically. 
     An image forming apparatus  100  according to the third exemplary embodiment employs two special colors: the transparent color (V) that gives a gloss to the image, and the green color (G) that widens the gamut of colors that are reproducible in the output image. 
     As illustrated in  FIG. 8 , toner-image-forming units  30  for the transparent color (V), yellow (Y), magenta (M), cyan (C), black (K), and green (G) are arranged side by side in the horizontal direction and in that order from the upstream side in the direction of rotation of the transfer belt  50  (see the arrow illustrated in  FIG. 8 ). 
     The toner-image-forming units  30 Y,  30 M,  30 C, and  30 K (exemplary first imaging units) form standard-color images in yellow (Y), magenta (M), cyan (C), and black (K), respectively. The toner-image-forming unit  30 G (an exemplary second imaging unit) forms a green (G) image. The toner-image-forming unit  30 V (an exemplary third imaging unit) forms a transparent image, i.e., a clear image. 
     The green toner T 3  used for forming the green (G) image has a volume-mean particle size of 7.5 μm. The standard toner T 1  used for forming each of the images in the standard colors of yellow (Y), magenta (M), cyan (C), and black (K) has a volume-mean particle size of 5.8 μm. The clear toner T 2  used for forming the clear (V) image has a volume-mean particle size of 3.8 μm. 
     In such a configuration, the toner images in the clear color (V), yellow (Y), magenta (M), cyan (C), black (K), and green (G) are superposed one on top of another in that order on the transfer belt  50  in the first transfer. When the set of toner images is transferred to a sheet member P for the second transfer, the order of the toner images thus superposed is reversed. That is, in the second transfer, the toner images are superposed on the sheet member P in the order of that in green (G), that in black (K), that in cyan (C), that in magenta (M), that in yellow (Y), and that in the clear color (V). 
     The green toner T 3  used for the green image formed on the sheet member P has a larger volume-mean particle size than the standard toners T 1  used for the standard-color images formed over the green image. Furthermore, the standard toners T 1  used for the standard-color images each have a larger volume-mean particle size than the clear toner T 2  used for the clear image formed over the standard-color images. In other words, the green toner T 3  having a relatively large volume-mean particle size is provided as a lower layer lying on the sheet member P, the standard toners T 1  having a relatively middle volume-mean particle size are provided as a middle layer lying over the lower layer, and the clear toner T 2  having a relatively small volume-mean particle size is provided as an upper layer lying over the middle layer. 
     Now, the set of toner images formed on the sheet member P by the image forming apparatus  100  will be described with reference to  FIGS. 9A and 9B . In  FIG. 9A , the difference in the size of toner particles forming the toner images is exaggerated for easy recognition. 
     In the image forming apparatus  100 , toner images are formed by using the toner-image-forming unit  30 G, the toner-image-forming unit  30 M, and the toner-image-forming unit  30 V, respectively. 
       FIG. 9A  is a sectional view illustrating the green image formed on the sheet member P, the standard-color image formed over the green image, and the clear image formed over the standard-color image, the three images being yet to be fixed to the sheet member P. As illustrated in  FIG. 9A , since the volume-mean particle sizes of the toners become smaller sequentially from the side of the sheet member P, particles of the toner in the upper layer do not push away particles of the toner in the middle layer, and particles of the toner in the middle layer do not push away particles of the toner in the lower layer. 
       FIG. 9B  is a sectional view illustrating the green image, the standard-color image, and the clear image that have been fixed to the sheet member P. As illustrated in  FIG. 9B , the green image, the standard-color image, and the clear image are superposed one on top of another in that order on the sheet member P. Thus, the occurrence of nonuniformity in the output image is suppressed. 
     While the present invention has been described in detail on the basis of some specific exemplary embodiments, the present invention is not limited to those exemplary embodiments. It is obvious to those skilled in the art that the present invention may be embodied in various other ways within the scope thereof. For example, while the above exemplary embodiments each concern a tandem-type image forming apparatus, the image forming apparatus according to the present invention may be a rotary-type image forming apparatus or another kind of tandem-type image forming apparatus that employs a method in which, for example, images on image carrying members are directly transferred to a sheet member. 
     In the first exemplary embodiment, the standard toners T 1  for the respective colors all have a volume-mean particle size of 5.8 Alternatively, the standard toners T 1  may have different volume-mean particle sizes. In that case, the clear toner T 2  only needs to have a volume-mean particle size smaller than the smallest one of the volume-mean particle sizes of the standard toners T 1 . 
     In the second exemplary embodiment, the standard toners T 1  for the respective colors all have a volume-mean particle size of 3.8 μm. Alternatively, the standard toners T 1  may have different volume-mean particle sizes. In that case, the green toner T 3  only needs to have a volume-mean particle size larger than the smallest one of the volume-mean particle sizes of the standard toners T 1 . 
     In the third exemplary embodiment, the standard toners T 1  for the respective colors all have a volume-mean particle size of 5.8 μm. Alternatively, the standard toners T 1  may have different volume-mean particle sizes. In that case, the green toner T 3  only needs to have a volume-mean particle size larger than the smallest one of the volume-mean particle sizes of the standard toners T 1 , and the clear toner T 2  only needs to have a volume-mean particle size smaller than the smallest one of the volume-mean particle sizes of the standard toners T 1 . 
     The special colors described in the above exemplary embodiment are only exemplary and may alternatively be any of other special colors such as violet, orange, and white. 
     In each of the second and third exemplary embodiments, the standard toner T 1  for the standard color of black (K) has a smaller volume-mean particle size than the green toner T 3  for the color of green (G). However, the standard toner T 1  for the standard color of black (K) does not necessarily have a smaller volume-mean particle size than the green toner T 3 . 
     Suppose that standard-color images and a clear image have been fixed to a sheet member P with a black (K) image forming the upper (or middle) layer. In such a case, even if any portions of the black image are in contact with the sheet member P, the level of nonuniformity in the output image is acceptable to the user. This is because the portions of the black image that are in contact with the sheet member P and portions of the black image that are on the green image have the same color tone. Since the hue of the black (K) color is very low compared with the hues of the other colors, the black (K) color tends to conceal other colors in the lower layers. Therefore, such portions are visually less noticeable as changes in color or as image nonuniformity. 
     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 embodiments were chosen and described in order to best explain the principles of the invention and its practical applications, thereby enabling others skilled in the art to understand the invention for various embodiments and with the various modifications as are suited to the particular use contemplated. It is intended that the scope of the invention be defined by the following claims and their equivalents.