Patent Publication Number: US-8983355-B2

Title: Image forming apparatus and image forming method

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based on and claims priority under 35 USC 119 from Japanese Patent Application No. 2012-067078 filed Mar. 23, 2012. 
     BACKGROUND 
     (i) Technical Field 
     The present invention relates to an image forming apparatus and an image forming method. 
     (ii) Related Art 
     An example of an image forming apparatus according to the related art forms a monochrome or full-color image by transferring a toner image formed on a photoconductor drum onto a recording medium with a transfer member and fixing the toner image. The transfer member transfers the toner image onto the recording medium directly or indirectly through an intermediate transfer belt. 
     SUMMARY 
     According to an aspect of the invention, there is provided an image forming apparatus including an image carrier that carries a first toner image, a transfer unit that transfers the first toner image on the image carrier onto a recording medium directly or through an intermediate transfer body, a fixing unit that fixes the first toner image that has been transferred onto the recording medium, and a glossiness-controlling unit that forms a second toner image made of clear toner on the recording medium in accordance with an amount of toner that forms the first toner image to be transferred onto the recording medium, thereby controlling a glossiness on a surface of the recording medium that has been subjected to the fixing performed by the fixing unit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       An exemplary embodiment of the present invention will be described in detail based on the following figures, wherein: 
         FIG. 1  illustrates the structure of an image forming apparatus according to an exemplary embodiment of the present invention; 
         FIG. 2  illustrates the structure of a part of the image forming apparatus according to the exemplary embodiment of the present invention; 
         FIG. 3  is a schematic diagram illustrating a color toner image that has been transferred onto a sheet of recording paper; 
         FIG. 4  is a graph showing the relationship between the total amount of color toners and glossiness; 
         FIG. 5  is a block diagram of a control circuit of the image forming apparatus according to the exemplary embodiment of the present invention; 
         FIGS. 6A and 6B  illustrate the contents of a bitmap memory; 
         FIG. 7  is a graph showing the variation in glossiness caused by clear toners on a surface of a sheet of recording paper; 
         FIG. 8  is a graph showing the variation in glossiness caused by a high-glossiness toner on a surface of a sheet of recording paper; 
         FIG. 9  is a graph showing the variation in glossiness caused by a low-glossiness toner on a surface of a sheet of recording paper; 
         FIG. 10  is a graph showing the glossiness of an image formed by the image forming apparatus according to the exemplary embodiment of the present invention; 
         FIG. 11  is a flowchart illustrating the operation of the image forming apparatus according to the exemplary embodiment of the present invention; 
         FIG. 12  illustrates a look-up table used to determine the amount of first clear toner; and 
         FIG. 13  illustrates another look-up table used to determine the amount of second clear toner. 
     
    
    
     DETAILED DESCRIPTION 
     An exemplary embodiment of the present invention will now be described with reference to the drawings. 
       FIGS. 1 and 2  illustrate an image forming apparatus  1  according to the exemplary embodiment.  FIG. 1  illustrates the overall structure of the image forming apparatus  1 , and  FIG. 2  illustrates an enlarged view of a part (for example, imaging devices) of the image forming apparatus  1 . 
     Overall Structure of Image Forming Apparatus 
     The image forming apparatus  1  according to the exemplary embodiment is, for example, a color printer. The image forming apparatus  1  includes plural imaging devices  10 , an intermediate transfer device  20 , a paper feeding device  50 , and a fixing device  40 . Each imaging device  10  forms a toner image developed with toner contained in developer  4 . The intermediate transfer device  20  carries toner images formed by the respective imaging devices  10  and transports the toner images to a second transfer position at which the toner images are transferred onto a sheet of recording paper  5 , which is an example of a recording medium, in a second transfer process. The paper feeding device  50  contains and transports the sheet of recording paper  5  to be supplied to the second transfer position of the intermediate transfer device  20 . The fixing device  40  fixes the toner images that have been transferred onto the sheet of recording paper  5  by the intermediate transfer device  20  in the second transfer process. 
     In the case where, for example, an image input device  60  that inputs a document image to be formed on the sheet of recording paper  5  is additionally provided, the image forming apparatus  1  may be configured as a color copier. Referring to  FIG. 1 , the image forming apparatus  1  includes a housing  1   a  including, for example, a supporting structural member and an external covering part. The one-dot chain line shows a transport path along which the sheet of recording paper  5  is transported in the housing  1   a.    
     Structure of Part of Image Forming Apparatus 
     The imaging devices  10  include six imaging devices  10 Y,  10 M,  10 C,  10 K,  10 S 1 , and  10 S 2 . The imaging devices  10 Y,  10 M,  10 C, and  10 K respectively form toner images of four colors, which are yellow (Y), magenta (M), cyan (C), and black (K). The imaging devices  10 S 1  and  10 S 2  respectively form two types of toner images of special colors S 1  and S 2 . The six imaging devices  10  (S 1 , S 2 , Y, M, C, and K) are arranged along a line in the inner space of the housing  1   a . The developers  4  (S 1  and S 2 ) of the special colors (S 1  and S 2 ) contain, for example, materials of colors which are difficult or impossible to be expressed by the above-described four colors. More specifically, toners of colors other than the four colors, toners having the same colors as the four colors but saturations different from those of the toners of four colors, clear toners that increase the glossiness, foaming toners used in Braille printing, fluorescent toners, etc., may be used. The imaging devices  10  (S 1 , S 2 , Y, M, C, and K) have a substantially similar structure, as described below, except for the type of the developer used therein. 
     As illustrated in  FIGS. 1 and 2 , each imaging device  10  (S 1 , S 2 , Y, M, C, or K) includes a photoconductor drum  11  that rotates, and devices described below are arranged around the photoconductor drum  11 . The devices include a charging device  12 , an exposure device  13 , a developing device  14  (S 1 , S 2 , Y, M, C, K), a first transfer device  15 , a pre-cleaning charging device  16 , a drum cleaning device  17 , and a electricity removing device  18 . The charging device  12  charges a peripheral surface (image carrying surface) of the photoconductor drum  11 , on which an image may be formed, to a certain potential. The exposure device  13  irradiates the charged peripheral surface of the photoconductor drum  11  with light LB based on image information (signal) to form an electrostatic latent image (for the corresponding color) having a potential difference. The developing device  14  (S 1 , S 2 , Y, M, C, or K) forms a toner image by developing the electrostatic latent image with toner contained in the developer  4  of the corresponding color (S 1 , S 2 , Y, M, C, or K). The first transfer device  15  performs a first transfer process in which the toner image is transferred onto the intermediate transfer device  20 . The pre-cleaning charging device  16  charges substances, such as toner, that remain on the image carrying surface of the photoconductor drum  11  after the first transfer process. The drum cleaning device  17  cleans the image carrying surface by removing the recharged substances. The electricity removing device  18  removes electricity from the image carrying surface of the photoconductor drum  11  after the cleaning process. 
     The photoconductor drum  11  includes a cylindrical or columnar base member that is grounded and a photoconductive layer (photosensitive layer) that is provided on the peripheral surface of the base member. The photoconductive layer is made of a photosensitive material and is provided with the image carrying surface. The photoconductor drum  11  is supported so as to be capable of rotating in the direction shown by arrow A when power is transmitted thereto from a rotation driving device (not shown). 
     The charging device  12  is a non-contact charging device, such as a corona discharger, and is arranged without contacting the photoconductor drum  11 . The charging device  12  includes a discharge member that receives a charging voltage. In the case where the developing device  14  performs reversal development, a voltage or current having the same polarity as the charging polarity of the toner supplied by the developing device  14  is supplied as the charging voltage. 
     The exposure device  13  forms the electrostatic latent image by irradiating the charged peripheral surface of the photoconductor drum  11  with light (arrowed dashed line) LB generated in accordance with the image information input to the image forming apparatus  1 . When forming the electrostatic latent image, the exposure device  13  receives the image information (signal) that is input to the image forming apparatus  1  by any method. 
     As illustrated in  FIG. 2 , each developing device  14  (S 1 , S 2 , Y, M, C, or K) includes a housing  140  having an opening and a chamber of the developer  4 . Two developing rollers  141  and  142 , two stirring-and-transporting members  143  and  144 , and a layer-thickness regulating member  145  are disposed in the housing  140 . The two developing rollers  141  and  142  hold the developer  4  and transport the developer  4  to respective developing areas in which the developing rollers  141  and  142  face the photoconductor drum  11 . The stirring-and-transporting members  143  and  144  are, for example, two screw augers that transport the developer  4  while stirring the developer  4  so that the developer  4  passes between the developing rollers  141  and  142 . The layer-thickness regulating member  145  regulates the amount (layer thickness) of the developer  4  held by the developing roller  142 . A developing voltage supplied from a power supply device (not shown) is applied between the photoconductor drum  11  and the developing rollers  141  and  142  of the developing device  14 . The developing rollers  141  and  142  and the stirring-and-transporting members  143  and  144  receive power from a rotation driving device (not shown) and rotates in a certain direction. Two-component developers containing nonmagnetic toner and magnetic carrier are used as the developers  4  (Y, M, C, and K) of the above-described four colors and the developers  4  (S 1  and S 2 ) of the two special colors. 
     The first transfer device  15  is a contact transfer device including a first transfer roller which rotates while contacting the peripheral surface of the photoconductor drum  11  and receives a first transfer voltage. A direct-current voltage having a polarity opposite to the charging polarity of the toner is supplied as the first transfer voltage from the power supply device (not shown). 
     As illustrated in  FIG. 2 , the drum cleaning device  17  includes a container-shaped body  170  that has an opening, a cleaning plate  171 , a rotating brush roller  172 , and a transporting member  173 . The cleaning plate  171  is arranged to contact the peripheral surface of the photoconductor drum  11  at a certain pressure after the first transfer process and clean the peripheral surface of the photoconductor drum  11  by removing substances such as residual toner therefrom. The rotating brush roller  172  is arranged to contact with the peripheral surface of the photoconductor drum  11  while rotating at a position upstream of the cleaning plate  171  in the rotation direction of the photoconductor drum  11 . The transporting member  173  is, for example, a screw auger that transports the substances such as toner that have been removed by the cleaning plate  171  to a collecting system (not shown). The cleaning plate  171  may be formed of a plate-shaped member (for example, a blade) made of rubber or the like. 
     As illustrated in  FIG. 1 , the intermediate transfer device  20  is disposed below the imaging devices  10  (S 1 , S 2 , Y, M, C, and K). The intermediate transfer device  20  basically includes an intermediate transfer belt  21 , plural belt support rollers  22  to  27 , a second transfer device  30 , and a belt cleaning device  28 . The intermediate transfer belt  21  rotates in the direction shown by arrow B while passing through a first transfer position, which is between the photoconductor drum  11  and the first transfer device  15  (first transfer roller). The belt support rollers  22  to  27  retain the intermediate transfer belt  21  in a desired position at the inner surface of the intermediate transfer belt  21  so that the intermediate transfer belt  21  is rotatably supported. The second transfer device  30  is disposed to oppose the belt support roller  26  that supports the intermediate transfer belt  21  at the outer-peripheral-surface (image-carrying-surface) side of the intermediate transfer belt  21 . The second transfer device  30  performs a second transfer process in which the toner images on the intermediate transfer belt  21  are transferred onto the sheet of recording paper  5 . The belt cleaning device  28  cleans the outer peripheral surface of the intermediate transfer belt  21  by removing substances such as toner and paper dust that remain on the outer peripheral surface of the intermediate transfer belt  21  after the intermediate transfer belt  21  has passed the second transfer device  30 . 
     The intermediate transfer belt  21  may be, for example, an endless belt made of a material obtained by dispersing a resistance adjusting agent, such as carbon black, in a synthetic resin, such as polyimide resin or polyamide resin. The belt support roller  22  serves as a driving roller. The belt support rollers  23 ,  25 , and  27  serve as driven rollers for retaining the position of the intermediate transfer belt  21 . The belt support roller  24  serves as a tension-applying roller. The belt support roller  26  serves as a back-up roller in the second transfer process. 
     As illustrated in  FIG. 1 , the second transfer device  30  includes a second transfer belt  31  and plural support rollers  32  to  36 . The second transfer belt  31  rotates in the direction shown by arrow C while passing through a second transfer position, which is on the outer-peripheral-surface side of the intermediate transfer belt  21  that is supported by the belt support roller  26  in the intermediate transfer device  20 . The support rollers  32  to  36  retain the second transfer belt  31  in a desired position at the inner surface of the second transfer belt  31  so that the second transfer belt  31  is rotatably supported. The second transfer belt  31  is, for example, an endless belt having substantially the same structure as that of the above-described intermediate transfer belt  21 . The belt support roller  32  is arranged so that the second transfer belt  31  is pressed at a certain pressure against the outer peripheral surface of the intermediate transfer belt  21  supported by the belt support roller  26 . The belt support roller  32  serves as a driving roller, and the belt support roller  36  serves as a tension-applying roller. The belt support roller  32  of the second transfer device  30  or the belt support roller  26  of the intermediate transfer device  20  receives a direct-current voltage having a polarity that is opposite to or the same as the charging polarity of the toner as a second transfer voltage. 
     The fixing device  40  includes a heating rotating body  42  and a pressing rotating body  43  that are arranged in a housing  41  having an inlet and an outlet for the sheet of recording paper  5 . The heating rotating body  42  includes a fixing belt that rotates in the direction shown by the arrow and that is heated by a heater so that the surface temperature thereof is maintained at a predetermined temperature. The pressing rotating body  43  is drum-shaped and contacts the heating rotating body  42  at a certain pressure substantially along the axial direction of the heating rotating body  42 , so that the pressing rotating body  43  is rotated. In the fixing device  40 , the contact portion in which the heating rotating body  42  and the pressing rotating body  43  contact each other serves as a fixing process unit that performs a certain fixing process (heating and pressing). 
     The paper feeding device  50  is disposed below the intermediate transfer device  20  and the second transfer device  30 . The paper feeding device  50  basically includes at least one paper container  51  that contains sheets of recording paper  5  of the desired size, type, etc., in a stacked manner and a transporting device  52  that feeds the sheets of recording paper  5  one at a time from the paper container  51 . The paper container  51  is, for example, attached to the housing  1   a  such that the paper container  51  may be pulled out therefrom at the front side (side that faces the user during operation) of the housing  1   a.    
     Plural pairs of paper transport rollers  53  to  57 , which transport each of the sheets of recording paper  5  fed from the paper feeding device  50  to the second transfer position, and a paper transport path including transport guide members (not shown) are provided between the paper feeding device  50  and the second transfer device  30 . The pair of paper transport rollers  57  that are disposed immediately in front of the second transfer position on the paper transport path serve as, for example, registration rollers for adjusting the time at which each sheet of recording paper  5  is to be transported. A paper transport device  58 , which may be belt-shaped, is provided between the second transfer device  30  and the fixing device  40 . The paper transport device  58  transports the sheet of recording paper  5  that has been transported from the second transfer belt  31  of the second transfer device  30  after the second transfer process to the fixing device  40 . A pair of paper discharge rollers  59  are disposed near a paper outlet formed in the housing  1   a . The pair of paper discharge rollers  59  discharge the sheet of recording paper  5  that has been subjected to the fixing process and transported from the fixing device  40  to the outside of the housing  1   a.    
     The image input device  60 , which is provided when the image forming apparatus  1  is formed as a color copier, is an image reading device that reads an image of a document  6  having the image information to be printed. The image input device  60  is arranged, for example, above the housing  1   a  as illustrated in  FIG. 1 . The image input device  60  basically includes a document receiving plate (platen glass)  61 , a light source  62 , a reflection mirror  63 , a first reflection mirror  64 , a second reflection mirror  65 , an image reading element  66 , and an imaging lens  67 . The document receiving plate  61  includes, for example, a transparent glass plate on which the document  6  having the image information to be read is placed. The light source  62  irradiates the document  6  placed on the document receiving plate  61  while moving. The reflection mirror  63  receives reflected light from the document  6  and reflects the light in a predetermined direction while moving together with the light source  62 . The first and second reflection mirrors  64  and  65  move at a predetermined speed by a predetermined distance with respect to the reflection mirror  63 . The image reading element  66  includes, for example, a charge coupled device (CCD) that receives and reads the reflected light from the document  6  and converts the reflected light into an electrical signal. The imaging lens  67  focuses the reflected light on the image reading element  66 . Referring to  FIG. 1 , the document receiving plate  61  is covered by an opening-closing covering part  68 . 
     The image information of the document  6  that has been read by the image input device  60  is input to an image processing device  70 , which subjects the image information to necessary image processing. The image input device  60  transmits the read image information of the document  6  to the image processing device  70  as, for example, red (R), green (G), and blue (B) three-color image data (for example, 8-bit data for each color). The image processing device  70  subjects the image data transmitted from the image input device  60  to predetermined image processing, such as shading correction, misregistration correction, brightness/color space conversion, gamma correction, frame erasing, and color/movement edition. The image processing device  70  converts the image signals obtained as a result of the image processing into image signals of the above-described four colors (Y, M, C, and K), and transmits the image signals to the exposure device  13 . The image processing device  70  also generates image signals for the two special colors (S 1  and S 2 ). 
     Operation of Image Forming Apparatus 
     A basic image forming operation performed by the image forming apparatus  1  will now be described. 
     First, an image forming operation for forming a full-color image by combining toner images of four colors (Y, M, C, and K) by using the four imaging devices  10  (Y, M, C, and K) will be described. 
     When the image forming apparatus  1  receives command information of a request for the image forming operation (printing), the four imaging devices  10  (Y, M, C, and K), the intermediate transfer device  20 , the second transfer device  30 , and the fixing device  40  are activated. 
     In each of the imaging devices  10  (Y, M, C, and K), first, the photoconductor drum  11  rotates in the direction shown by arrow A and the charging device  12  charges the surface of the photoconductor drum  11  to a certain potential with a certain polarity (negative polarity in the present exemplary embodiment). Subsequently, the exposure device  13  irradiates the charged surface of the photoconductor drum  11  with the light LB based on the image signal obtained by converting the image information input to the image forming apparatus  1  into a component of the corresponding color (Y, M, C, or K). As a result, an electrostatic latent image for the corresponding color having a certain potential difference is formed on the surface of the photoconductor drum  11 . 
     After that, each of the developing devices  14  (Y, M, C, and K) supplies the toner of the corresponding color (Y, M, C, or K), charged with a certain polarity (negative polarity), from the developing rollers  141  and  142  to the electrostatic latent image of the corresponding color formed on the photoconductor drum  11 . The toner electrostatically adheres to the electrostatic latent image, so that the electrostatic latent image is developed. As a result of the developing process, the electrostatic latent images for the respective colors formed on the photoconductor drums  11  are visualized as toner images of the four colors (Y, M, C, and K) developed with the toners of the respective colors. 
     When the toner images of the respective colors formed on the photoconductor drums  11  of the imaging devices  10  (Y, M, C, and K) reach the respective first transfer positions, the first transfer devices  15  perform the first transfer process so that the toner images of the respective colors are successively transferred, in a superimposed manner, onto the intermediate transfer belt  21  of the intermediate transfer device  20  that rotates in the direction of arrow B. 
     In each imaging device  10 , after the first transfer process, the pre-cleaning charging device  16  recharges the substances, such as toner, that remain on the surface of the photoconductor drum  11  after the first transfer process. Subsequently, the drum cleaning device  17  cleans the surface of the photoconductor drum  11  by scraping off the recharged substances, and the electricity removing device  18  removes the electricity from the cleaned surface of the photoconductor drum  11 . Thus, the imaging device  10  is set to a standby state for the next imaging operation. 
     In the intermediate transfer device  20 , the intermediate transfer belt  21  rotates so as to transport the toner images that have been transferred onto the intermediate transfer belt  21  by the first transfer process to the second transfer position. The paper feeding device  50  feeds a sheet of recording paper  5  to the paper transport path in accordance with the imaging operation. In the paper transport path, the pair of paper transport rollers  57 , which serve as registration rollers, transport the sheet of recording paper  5  to the second transfer position in accordance with the transfer timing. 
     At the second transfer position, the second transfer device  30  performs the second transfer process in which the toner images on the intermediate transfer belt  21  are simultaneously transferred onto the sheet of recording paper  5 . In the intermediate transfer device  20  after the second transfer process, the belt cleaning device  28  cleans the surface of the intermediate transfer belt  21  by removing the substances, such as toner, that remain on the surface after the second transfer process. 
     The sheet of recording paper  5 , onto which the toner images have been transferred by the second transfer process, is released from the intermediate transfer belt  21  and from the second transfer belt  31  and transported to the fixing device  40  by the paper transport device  58 . In the fixing device  40 , the sheet of recording paper  5  after the second transfer process is guided through the contact portion between the heating rotating body  42  and the pressing rotating body  43  that rotate. Thus, a fixing process (heating and pressing) is performed so that the unfixed toner images are fixed to the sheet of recording paper  5 . In the case where the image forming operation is performed to form an image only on one side of the sheet of recording paper  5 , the sheet of recording paper  5  that has been subjected to the fixing process is discharged to, for example, a discharge container (not illustrated) disposed outside the housing  1   a  by the paper discharge rollers  59 . 
     As a result of the above-described operation, the sheet of recording paper  5  on which a full-color image is formed by combining toner images of four colors is output. 
     Next, the case will be described in which special-color toner images are additionally formed by using the developers of the special colors S 1  and S 2  in the above-described normal image forming operation performed by the image forming apparatus  1 . 
     In this case, first, the imaging devices  10 S 1  and  10 S 2  perform an operation similar to the imaging operation performed by the imaging devices  10  (Y, M, C, and K). Accordingly, special-color toner images (S 1  and S 2 ) are formed on the photoconductor drums  11  of the imaging devices  10 S 1  and  10 S 2 . Subsequently, similar to the manner in which the toner images of the four colors are processed in the above-described image forming operation, the special-color toner images formed by the imaging devices  10 S 1  and  10 S 2  are transferred onto the intermediate transfer belt  21  of the intermediate transfer device  20  in the first transfer process. Then, in the second transfer process, the second transfer device  30  transfers the special-color toner images from the intermediate transfer belt  21  onto the sheet of recording paper  5  together with the toner images of the other colors. Lastly, the sheet of recording paper  5 , onto which the special-color toner images and the toner images of the other colors have been transferred in the second transfer process, is subjected to the fixing process performed by the fixing device  40  and discharged to the outside of the housing  1   a.    
     As a result of the above-described operation, the sheet of recording paper  5  is output on which the two special-color toner images overlap with a part or the entirety of the full-color image formed by combining the toner images of four colors together. 
     In the case where the image forming apparatus  1  is equipped with the image input device  60  and serves as a color copier, a basic image forming operation is performed as follows. 
     That is, in this case, when the document  6  is set to the image input device  60  and command information of a request for the image forming operation (copying) is input, the image input device  60  reads the document image from the document  6 . The information of the read document image is subjected to the above-described image processing performed by the image processing device  70 , so that the image signals are generated. The image signals are transmitted to the exposure devices  13  of the imaging devices  10  (S 1 , S 2 , Y, M, C, and K). Accordingly, each imaging device  10  forms an electrostatic latent image and a toner image based on the image information of the document  6 . After that, an operation similar to the above-described image forming operation (printing) is performed and the sheet of recording paper  5  on which an image obtained by combining the toner images together is formed is output. 
     Structure of Glossiness Controller 
     The image forming apparatus  1 , which is structured as described above, further includes a glossiness controller. The glossiness controller reduces the occurrence of differences (unevenness) in glossiness of an image formed on the sheet of recording paper  5 , which serves as a recording medium, over the surface of the sheet of recording paper  5 , the image being formed of the toner images of four colors, which are yellow (Y), magenta (M), cyan (C), and black (K). A part of the glossiness controller includes the imaging devices  10 S 1  and  10 S 2  that use two types of clear toners having different glossinesses. 
     The developers  4  (S 1  and S 2 ) of the special colors used in the imaging devices  10 S 1  and  10 S 2  may be, for example, two types of clear toners used to correct the differences in glossiness over the surface of the recording medium on which an image is formed by using the above-described four colors. More specifically, the developers  4  (S 1  and S 2 ) may be a developer  4  (S 1 ) of a special color including a first glossiness-correcting clear toner having a relatively high glossiness and a developer  4  (S 2 ) of a special color including a second glossiness-correcting clear toner having a relatively low glossiness. The imaging devices  10 S 1  and  10 S 2 , which use the developers  4  (S 1  and S 2 ) of the special colors, form a part of a glossiness-controlling unit. Of the imaging devices  10 , the imaging devices  10 S 1  and  10 S 2  are arranged at the most upstream positions along the moving direction of the intermediate transfer belt  21 . The clear toners used in the imaging devices  10 S 1  and  10 S 2  are obtained by removing colorants from the yellow (Y), magenta (M), or cyan (C) color toner. The first glossiness-correcting clear toner having a relatively high glossiness and the second glossiness-correcting clear toner having a relatively low glossiness are formed by adjusting the amounts of crystalline polymer in the toners so that the toners form images having different glossinesses after the fixing process. When, for example, the amount of crystalline polymer in the yellow (Y), magenta (M), and cyan (C) color toners is 8 weight percent, the amount of crystalline polymer in the second glossiness-correcting clear toner having a relatively low glossiness may be set to, for example, 4 weight percent and the amount of crystalline polymer in the first glossiness-correcting clear toner having a relatively high glossiness may be set to, for example, 10 weight percent. 
     Here, for example, coated paper (OKTC) having a basis weight of 127 gms is used as the sheet of recording paper  5 , and images are formed by using toners of three colors, which are yellow (Y), magenta (M), and cyan (C) excluding black (K), while changing image densities Cin of the yellow (Y), magenta (M), and cyan (C) color toners in the range of 0% to 100%, as illustrated in  FIG. 3 . According to the experiments conducted by the present inventor, it has been found that the glossiness of the sheet of recording paper  5  after the toner images have been fixed thereto varies as in  FIG. 4  in accordance with the total color-toner density Cin (%), which is the sum of the image densities of the color toners. To measure the glossiness of the color images, “Micro-gloss 4460” produced by BYK Gardner Inc. is used. The glossiness is measured at a measurement angle of 60°. 
     The full-color images formed on the sheet of recording paper  5 , which is coated paper, have differences (unevenness) in glossiness in the range of about 30, which is the glossiness of the coated paper itself, to as high as about 80 over the surface of the recording paper  5 . The glossiness varies in accordance with the total toner image density ΣCin, which is the sum of the image densities Cin of the toner images of three colors, which are yellow (Y), magenta (M), and cyan (C). Thus, the glossiness on the surface of the sheet of recording paper  5  largely varies, in other words, glossiness unevenness occurs, in accordance with the total toner density on the recording paper  5 . 
     Accordingly, in this exemplary embodiment, the glossiness-controlling unit, which controls the glossiness on the surface of the sheet of recording paper  5  that has been subjected to the fixing process performed by the fixing device  40 , is provided. The glossiness-controlling unit controls the glossiness by forming images of clear toners on the sheet of recording paper  5  in accordance with the total amount of toner in the color toner images to be transferred onto the sheet of recording paper  5 . 
       FIG. 5  is a block diagram illustrating a control circuit of the image forming apparatus  1 . 
     Referring to  FIG. 5 , a central processing unit (CPU)  100  forms a part of the glossiness controller. The CPU  100  controls not only the overall image forming operation of the image forming apparatus  1  but also a clear-toner-image forming operation. In the clear-toner-image forming operation, amounts of toner in clear toner images, which are formed by the imaging devices  10  of the special colors S 1  and S 2  by using the clear toners, are calculated for each pixel and the clear toner images are formed accordingly. The CPU  100  controls the image forming operation and the clear-toner-image forming operation on the basis of programs stored in advance in a read only memory (ROM)  101  while referring as appropriate to parameters stored in a random access memory (RAM)  102 . 
     The image processing device  70  receives image signals from a personal computer (not shown) or the image input device  60  and subjects the image signals to image processing. 
     A user may operate a user interface  103  to set the size and type of the sheet of recording paper  5  on which an image is to be printed or a desired value of glossiness in the process of printing an image on the sheet of recording paper  5 . 
     The image signals of each page for yellow (Y), magenta (M), cyan (C), and black (K) toners are divided into pixels in a bitmap memory  104 , the image signals being output from the image processing device  70 . The bitmap memory  104  is, for example, provided for each color of yellow (Y), magenta (M), cyan (C), and black (K). 
     The six imaging devices  10 S 1 ,  10 S 2 ,  10 Y,  10 M,  10 C, and  10 K include the imaging devices  10 S 1  and  10 S 2  that respectively form the two types of toner images of the special colors S 1  and S 2  and the imaging devices  10 Y,  10 M,  10 C, and  10 K that respectively form the toner images of four colors, which are yellow (Y), magenta (M), cyan (C), and black (K). 
     Operation of Glossiness Controller 
     The operation of the glossiness controller will now be described. 
     Referring to  FIG. 5 , the image processing device  70  generates the image signals output to the exposure devices  13  included in the four imaging devices  10  (Y, M, C, K) for yellow (Y), magenta (M), cyan (C), and black (K). 
     Referring to  FIG. 5 , the CPU  100  calculates density information of the toner of each color for each pixel on the basis of the image signals output from the image processing device  70  to the exposure devices  13 . When the image signals are output to the exposure devices  13  of the imaging devices  10  (Y, M, C, and K), the CPU  100  creates a bitmap in accordance with the image resolution by using the bitmap memory  104 , as illustrated in  FIGS. 6A and 6B , and determines the image density Cin(%) of each of the yellow (Y), magenta (M), and cyan (C) toners for each pixel G. 
     Referring to  FIG. 6A , assume that the image densities Cin(%) of yellow (Y), magenta (M), and cyan (C) at the pixel G 11  in the first line and first column are Cin=10% for yellow (Y), Cin=20% for magenta (M), and Cin=10% for cyan (C). In this case, the total image density Cin(%) of yellow (Y), magenta (M), and cyan (C) at the pixel G 11  in the first line and first column is calculated as 10+20+10=40%, as illustrated in  FIG. 6B . The CPU  100  calculates the total image densities Cin 11  to Cin mn  (%) of color toners for all of the pixels G in each page of the recording paper  5 . 
     Referring to  FIG. 11 , the CPU  100  refers to a look-up table (LUT) (not shown) and calculates the total amount of color toners (Tcolor) for each pixel on the basis of the total image density Cin(%) of yellow (Y), magenta (M), and cyan (C) (step  101 ). Calculations described below are performed for each pixel by the CPU  100 . 
     When the total amount of color toners is determined, the CPU  100  refers to a graph or table (not shown) showing the relationship between the total amount of color toners and glossiness and calculates the glossiness obtained when the color toners are fixed to the sheet of recording paper  5  on the basis of the total image density Cin(%) of the color toners. 
     The CPU  100  may determine the glossiness of the toner images fixed to the sheet of recording paper  5  directly on the basis of the total image density Cin(%) of the color toners, as illustrated in  FIG. 4 , without converting the total image density Cin(%) into the total amount of color toners. 
     Next, the CPU  100  determines the target glossiness of an image, such as a full-color image, formed by fixing the toner images to the sheet of recording paper  5 . The target glossiness may either be set by the user through the user interface  103  or determined automatically by the CPU  100  on the basis of the glossiness of the sheet of recording paper  5  that is used. In the present exemplary embodiment, the recording paper  5  is OKTC paper, which is coated paper having a surface glossiness of about 30 and a basis weight of 127 gms. The target glossiness is set to 60, which is obtained by adding a predetermined value α (for example, 30) to the glossiness of the recording paper  5 . 
     As illustrated in  FIG. 11 , the CPU  100  calculates the glossinesses (Tgloss) of the clear toners that are used (step  102 ), and also calculates the average amount of toner (Tave) in the toner images that form the image on the surface of the sheet of recording paper  5  (step  103 ). 
     In step  104  of  FIG. 11 , to achieve the target glossiness Ctarget ( 60 ), the CPU  100  calculates the total amount of clear toners (Tclear) for each pixel G by subtracting the total amount of color toners (Tcolor) from the average amount of toner (Tave) in the toner images that form the image on the surface of the sheet of recording paper  5  as follows:
 
Tclear=Tave−Tcolor
 
     The CPU  100  determines whether the calculated total amount of clear toners Tclear is a positive value or a negative value (step  105 ). If the calculated total amount of clear toners Tclear is a positive value (Tclear&gt;0), only the first clear toner having a relatively high glossiness is used. The amount of first clear toner to be used to achieve the target glossiness Ctarget is determined from a first look-up table (Lut 1 ) illustrated in  FIG. 12  (step  106 ). 
     If the calculated total amount of clear toners Tclear is less than or equal to zero, the CPU  100  uses only the second clear toner having a relatively low glossiness. The amount of second clear toner to be used to achieve the target glossiness Ctarget is determined from a second look-up table (Lut 2 ) illustrated in  FIG. 13  (step  107 ). 
     Here, assume that coated paper (OKTC paper) having a basis weight of 127 gms is used as the recording paper  5  and toner images that are formed using only the first high-glossiness clear toner and the second low-glossiness clear toner having different glossinesses are fixed to a sheet of recording paper  5 . In this case, when the image densities Cin of the toners are varied in the range of 0% to 100%, the glossiness on the surface of the sheet of recording paper  5  varies, for example, as illustrated in  FIG. 7 . As the image density Cin of the first high-glossiness clear toner increases, the glossiness on the surface of the sheet of recording paper  5  increases from about 30 to about 70. As the image density Cin of the second low-glossiness clear toner increases, the glossiness on the surface of the sheet of recording paper  5  once decreases to about 30 or less. Then, when the image density Cin exceeds 50%, the glossiness on the surface of the sheet of recording paper  5  slightly increases to about 40. 
     In the case where the toner images formed by using the first high-glossiness clear toner and the second low-glossiness clear toner are superimposed on the toner images formed by using the color toners, the glossiness on the surface of the sheet of recording paper  5  varies, for example, as illustrated in  FIGS. 8 and 9 . In the case where the first high-glossiness clear toner is used, when Cin=0% and the first high-glossiness clear toner is not used, the glossiness on the surface of the sheet of recording paper  5  increases along with the total amount of color toners, as illustrated in  FIG. 8 . When the amount of first high-glossiness clear toner is increased to Cin=50% and Cin=100%, the glossiness is largely increased in accordance with the amount of high-glossiness clear toner in a low-density range in which the total amount of color toners is 100% or less. In a high-density range in which the total amount of color toners is 200% or more, the glossiness is maintained at a high level almost irrespective of the amount of high-glossiness clear toner. 
     In the case where the second low-glossiness clear toner is used, when Cin=0% and the second low-glossiness clear toner is not used, the glossiness on the surface of the sheet of recording paper  5  increases along with the total amount of color toners, as illustrated in  FIG. 9 . When the amount of second low-glossiness clear toner is increased to Cin=50% and Cin=100%, the glossiness is largely reduced as the amount of low-glossiness clear toner is increased to Cin=50% and Cin=100% in a high-density range in which the total amount of color toners is 150% or more. When the amount of second low-glossiness clear toner is Cin=100%, the glossiness may be set to approximately 20 irrespective of the total amount of color toners. 
     Referring to  FIG. 6B , the bitmap memory  104  stores a bitmap of the image to be printed on the sheet of recording paper  5 . The CPU  100  determines the amount of first clear toner and the amount of second clear toner for each pixel G of the image to be printed, and outputs image signals representing the amounts of first and second clear toners for each pixel G to the exposure devices  13  in the imaging devices  10  for the special colors S 1  and S 2 . Then, the printing operation is started in the same manner as in the normal case. 
     The printing operation is performed as follows. That is, referring to  FIG. 1 , in the imaging device  10  for the special color S 1  using the first clear toner having a relatively high glossiness, the surface of the photoconductor drum  11  is charged by the charging device  12 , and is subjected to image exposure for each pixel G on the basis of the total amount of clear toners determined by the CPU  100 . The electrostatic latent image formed on the surface of the photoconductor drum  11  is visualized by the developing device  14 , and is transferred onto the intermediate transfer belt  21  in the first transfer process. 
     Next, in the imaging device  10  for the special color S 2  using the second clear toner having a relatively low glossiness, the surface of the photoconductor drum  11  is charged by the charging device  12 , and is subjected to image exposure for each pixel G on the basis of the total amount of clear toners determined by the CPU  100 . The electrostatic latent image formed on the surface of the photoconductor drum  11  is visualized by the developing device  14 , and is transferred onto the intermediate transfer belt  21  in the first transfer process. 
     Subsequently, the imaging devices  10  (Y, M, C, and K) for yellow (Y), magenta (M), cyan (C), and black (K) successively form the toner images of the respective colors, and the toner images are transferred onto the intermediate transfer belt  21  in a superimposed manner in the first transfer process. 
     Thus, the toner images of the respective colors, which are the special colors S 1  and S 2 , yellow (Y), magenta (M), cyan (C), and black (K), are transferred onto the intermediate transfer belt  21  in a superimposed manner in the first transfer process. Subsequently, the toner images are simultaneously transferred onto the sheet of recording paper  5  from the intermediate transfer belt  21  at the second transfer position in the second transfer process. After that, the sheet of recording paper  5  is transported to the fixing device  40 , where the fixing process using heat and pressure is performed, and discharged to the outside of the image forming apparatus  1  by the pair of paper discharge rollers  59 . 
     The glossiness of the thus-obtained full-color image is shown in  FIG. 10 . Referring to  FIG. 10 , in the pixel area in which the total amount of yellow (Y), magenta (M), and cyan (C) color toners is relatively large and the glossiness obtained when the color toners are fixed to the sheet of recording paper  5  is higher than the target glossiness, which is  60 , a toner image is formed by using the second low-glossiness clear toner in accordance with the total amount of color toners. Thus, the glossiness in this pixel area is controlled to approximately 60. 
     In addition, referring to  FIG. 10 , in the pixel area in which the total amount of yellow (Y), magenta (M), and cyan (C) color toners is relatively small and the glossiness obtained when the color toners are fixed to the sheet of recording paper  5  is lower than or equal to the target glossiness, which is 60, a toner image is formed by using the first high-glossiness clear toner in accordance with the total amount of color toners. Thus, the glossiness in this pixel area is increased to approximately 60. 
     As is clear from  FIG. 10 , the glossiness of the full-color image is approximately 60 over the entire area of the surface of the sheet of recording paper  5 . Thus, an image in which differences (unevenness) in glossiness are extremely small and which has appropriate glossiness characteristics may be formed on the surface of the sheet of recording paper  5 . In a blank area in which no toner image is formed using the color toners and the total amount of color toners is around zero, the glossiness may be increased to approximately 60 by forming a toner image using the first high-glossiness clear toner. 
     In the above-described exemplary embodiment, two types of clear toners having different glossinesses are used. However, the number of types of clear toners used may, of course, instead be one. 
     In addition, in the above-described exemplary embodiment, a so-called tandem image forming apparatus which includes plural imaging devices is described. However, the image forming apparatus, of course, may instead be a so-called 4-cycle image forming apparatus including only one photoconductor drum. 
     In addition, in the above-described exemplary embodiment, the image forming apparatus includes an intermediate transfer device. However, the image forming apparatus may instead be structured such that the toner images are directly transferred onto a recording medium that is transported by a transport belt or the like. The foregoing description of the exemplary embodiment 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 embodiment was 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.