Patent Publication Number: US-10782631-B2

Title: Image forming apparatus

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The entire disclosure of Japanese Patent Application No. 2019-012400, filed on Jan. 28, 2019, is incorporated herein by reference in its entirety. 
     BACKGROUND 
     1. Technological Field 
     The present invention relates to an image forming apparatus. 
     2. Description of the Related Art 
     There is a demand for a fixing device that can stably fix a high-quality image on paper. In an electrophotographic image forming apparatus, paper on which a toner image has been formed by an image forming unit is caused to pass through a fixing nip portion of a fixing device to be heated and pressurized, thereby fixing the toner image on the paper. The fixing nip portion is formed, for example, between a fixing belt, disposed around a heating roller and a fixing roller, and a pressure roller in a configuration having the fixing belt, and is formed between the fixing roller and the pressure roller in a configuration having no fixing belt 
     In relation to such a configuration, JP 2014-81610 A discloses a technique that provides a velocity difference between a surface of a pressure roller and a surface of a fixing belt to generate a shearing force between an image face on paper and the fixing belt, thereby preventing a phenomenon (gloss memory) in which the influence of a previous fixing process appears gloss unevenness. 
     SUMMARY 
     However, the image quality sometimes deteriorate when density unevenness of an image occurs although the shearing force applied between the image face on the paper and the fixing belt is advantageous in terms of controlling the glossiness. 
     The present invention has been made in view of the above circumstances, and an object thereof is to provide an image forming apparatus capable of preventing or suppressing the occurrence of density unevenness even when a shearing force is applied between an image face on paper and a fixing belt. 
     To achieve at least one of the above-mentioned objects, according to an aspect of the present invention, an image forming apparatus reflecting one aspect of the present invention comprises: an image former that forms a toner image constituted by a plurality of dots each containing a toner on a sheet; a fixer that includes a fixing member moving at a first velocity and a pressure member forming a fixing nip portion with the fixing member and moving at a second velocity, and fixes the toner image on the sheet at the fixing nip portion; and a controller that determines a toner amount such that a toner amount distribution is different in each of the dots before fixing when the first velocity and the second velocity at the fixing nip portion are different. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The advantages and features provided by one or more embodiments of the invention will become more fully understood from the detailed description given hereinbelow and the appended drawings which are given by way of illustration only, and thus are not intended as a definition of the limits of the present invention. 
         FIG. 1  is a schematic cross-sectional view illustrating a configuration of an image forming apparatus according to one embodiment; 
         FIG. 2  is a schematic block diagram illustrating the configuration of the image forming apparatus illustrated in  FIG. 1 ; 
         FIG. 3  is a schematic view illustrating the main configuration of a fixing unit illustrated in  FIG. 1 ; 
         FIG. 4  is a conceptual view for describing a cross-sectional shape of a toner image and a cross-sectional shape of a dot after normal fixing; 
         FIG. 5  is a schematic view illustrating a relationship between the magnitude of a shearing force applied to a toner particle of the toner image and the deformation of the toner particle; 
         FIG. 6  is a graph illustrating a relationship between the shearing force applied to the toner particle of the toner image, a deformation amount and glossiness of the toner particle; 
         FIG. 7A  is a photograph of a top view of a dot image on paper when normal fixing is performed; 
         FIG. 7B  is a photograph of a top view of a dot image on paper when a toner is fixed by applying a shearing force (brake); 
         FIG. 8  is a graph illustrating a result of measurement of a height of a dot with respect to a distance of the dot in a paper passage direction; 
         FIG. 9  is a schematic view illustrating a relationship between a dot image pattern and density unevenness; 
         FIG. 10  is a graph illustrating a relationship between a dot slope and a velocity difference; 
         FIG. 11A  is a schematic view for describing a marking applied on a fixing belt; 
         FIG. 11B  is a schematic view for describing a marking trace attached to paper after fixing; 
         FIG. 12A  is a conceptual view for describing a cross-sectional shape of a toner image formed by an image forming unit of the present embodiment and a cross-sectional shape of a dot fixed by applying a shearing force to a toner; 
         FIG. 12B  is a conceptual view for describing a cross-sectional shape of a toner image formed by the image forming unit of the present embodiment and a cross-sectional shape of a dot fixed by applying a shearing force to a toner; 
         FIG. 13A  is a conceptual view for describing an example in which a laser output of the image forming unit is changed as means for changing the density of the toner image; 
         FIG. 13B  is a conceptual view for describing another example in which the laser output of the image forming unit is changed as means for changing the density of the toner image; and 
         FIG. 14  is a conceptual view for describing a change amount in the density change of the toner image. 
     
    
    
     DETAILED DESCRIPTION OF EMBODIMENTS 
     Hereinafter, one or more embodiments of the present invention will be described with reference to the drawings. However, the scope of the invention is not limited to the disclosed embodiments. The same elements will be denoted by the same reference signs in the description of the drawings, and redundant descriptions thereof will be omitted. In addition, dimensional ratios in the drawings are exaggerated for convenience of the description, and are different from the actual ratios in some cases. 
     One Embodiment 
     &lt;Image Forming Apparatus  100 &gt; 
       FIG. 1  is a schematic cross-sectional view illustrating a configuration of an image forming apparatus  100  according to one embodiment, and  FIG. 2  is a schematic block diagram illustrating the configuration of the image forming apparatus  100 . 
     The image forming apparatus  100  is called a tandem color image forming apparatus, reads an image from an original, and forms (prints) the read image on a sheet (for example, paper). Note that examples of the sheet can include one made of resin, one whose surface is coated with resin, and the like in addition to the paper. Hereinafter, a case where the paper is used as the sheet will be exemplified. In addition, the image forming apparatus  100  receives a print job including print data and print setting information in a PDL (Page Description Language) format from an external client terminal via a network, and forms an image on the paper based on the print job. The client terminal can be, for example, a personal computer, a tablet terminal, a smartphone, or the like. 
     As illustrated in  FIG. 2 , the image forming apparatus  100  includes an image reading unit  110 , an image processing unit  120 , an image forming unit (image former)  130 , a paper feeding unit  140 , a paper conveying unit  150 , a fixing unit (fixer)  200 , a communication unit  300 , an operation display unit  400 , and a control unit (controller)  500 . These components are connected to each other via an internal bus  101  so as to be capable of communicating with each other. 
     The image reading unit  110  reads an original and outputs an image information signal. The original placed on an original table  111  is scanned and exposed by an optical system of a scanning exposure device of an image reading device  112  and read by a line image sensor. The photoelectrically converted image information signal is subjected to analog processing, A/D conversion, shading correction, image compression processing, and the like in the image processing unit  120 , and then, is input to optical writing units  3 Y,  3 M,  3 C, and  3 K (to be described later) of the image forming unit  130  as print image data. 
     The image forming unit  130  forms an image (toner image) on the paper based on the print image data using a known image creation process such as an electrophotographic scheme including charging, exposing, developing, and transferring steps. The image forming unit  130  includes four sets of a subunit  13 Y, a subunit  13 M, a subunit  13 C, and a subunit  13 K that form yellow (Y), magenta (M), cyan (C), and black (K) images, respectively. 
     The subunit  13 Y includes a photosensitive drum  1 Y and a charging unit  2 Y, an optical writing unit  3 Y, a developing device  4 Y, and a drum cleaner  5 Y arranged around the photosensitive drum  1 Y. 
     Similarly, the subunit  13 M includes a photosensitive drum  1 M and a charging unit  2 M, an optical writing unit  3 M, a developing device  4 M, and a drum cleaner  5 M arranged around the photosensitive drum  1 M; the subunit  13 C includes a photosensitive drum  1 C and a charging unit  2 C, an optical writing unit  3 C, a developing device  4 C, and a drum cleaner  5 C arranged around the photosensitive drum  1 C; and the subunit  13 K includes a photosensitive drum  1 K and a charging unit  2 K, an optical writing unit  3 K, a developing device  4 K, and a drum cleaner  5 K arranged around the photosensitive drum  1 K. 
     The respective photosensitive drums  1 Y,  1 M,  1 C, and  1 K, the respective charging units  2 Y,  2 M,  2 C, and  2 K, the respective optical writing units  3 Y,  3 M,  3 C, and  3 K, and the respective developing devices  4 Y,  4 M,  4 C, and  4 K, and the respective drum cleaners  5 Y,  5 M,  5 C, and  5 K in the subunits  13 Y,  13 M,  13 C, and  13 K have the same content, respectively. Hereinafter, reference signs Y, M, C, and K will not be used unless particularly distinguished. 
     The image forming unit  130  writes the print image data on the photosensitive drum  1  with a laser provided in the optical writing unit  3 , and forms a latent image based on the print image data on the photosensitive drum  1 . Then, the latent image is developed by the developing device  4  to form a toner image, which is a visible image, on the photosensitive drum  1 . 
     The yellow (Y), magenta (M), cyan (C), and black (K) images are formed, respectively, on the photosensitive drums  1 Y,  1 M,  1 C, and  1 K of the subunits  13 Y,  13 M,  13 C, and  13 K. 
     An intermediate transfer belt  6  is wound around a plurality of rollers to be supported so as to be capable of travel. The respective color toner images formed by the subunits  13 Y,  13 M,  13 C, and  13 K are sequentially transferred by primary transfer units  7 Y,  7 M,  7 C, and  7 K onto the traveling intermediate transfer belt  6 , thereby forming a color image in which Y (yellow), M (magenta), C (cyan), and K (black) color layers are superimposed on each other. 
     The paper feeding unit  140  supplies paper as a recording material to the image forming unit  130 . The paper feeding unit  140  includes an upper tray  141 , a middle tray  142 , and a lower tray  143 , and the respective trays accommodate paper of different sizes, for example, A4 size, A3 size, and the like. 
     The paper conveying unit  150  conveys paper. The paper supplied from the upper tray  141 , the middle tray  142 , or the lower tray  143  is conveyed to a secondary transfer unit  7 A via a resist roller  151 , and the color image on the intermediate transfer belt  6  is transferred onto the paper. 
     In addition, the paper conveying unit  150  includes a paper reversing unit  152  so that it is possible to guide the paper that has been subjected to fixing to the paper reversing unit  152 , to reverse the front and back and discharge the reversed paper or to form an image on both sides of the paper. 
     The fixing unit  200  fixes the color toner image formed on the paper by the image forming unit  130  onto the paper at a fixing nip portion. The paper on which the color image has been fixed is discharged to the outside of the image forming apparatus  100  through a paper discharge unit  153 . Details of the fixing unit  200  will be described later. 
     The communication unit  300  is connected to a client terminal such as a personal computer via a network and transmits and receives data such as a print job. 
     The operation display unit  400  includes an input unit and an output unit. The input unit includes, for example, a keyboard and a touch panel, and is used by a user to perform various instructions (inputs) such as character input, various settings, and print instructions. In addition, the output unit includes a display, and is used to present a device configuration, an execution status of the print job, an occurrence status of an abnormality (jam) during paper conveyance, and the like to the user. 
     The control unit  500  controls the image reading unit  110 , the image processing unit  120 , the image forming unit  130 , the paper feeding unit  140 , the paper conveying unit  150 , the fixing unit  200 , the communication unit  300 , and the operation display unit  400 . The control unit  500  includes a CPU  510 , an auxiliary storage unit  520 , a RAM  530 , and a ROM  540 . 
     The CPU  510  executes a control program for the image forming apparatus. The control program is stored in the auxiliary storage unit  520  and loaded into the RAM  530  when executed by the CPU  510 . The auxiliary storage unit  520  includes a large-capacity storage device, for example, a hard disk drive, a flash memory, and the like. The RAM  530  stores a calculation result or the like accompanying the execution of the CPU  510 . The ROM  540  stores various parameters, various programs, and the like. The CPU  510  executes the control program and to realize various functions. 
     &lt;Fixing Unit  200 &gt; 
     Next, a specific configuration of the fixing unit  200  will be described with reference to  FIG. 3 .  FIG. 3  is a schematic view illustrating the main configuration of the fixing unit  200  illustrated in  FIG. 1 . 
     The fixing unit  200  includes a non-contact temperature sensor  201 , a heating roller  210 , a fixing belt  220 , a fixing roller  230 , a pressure roller  240 , a first drive motor  250 , a second drive motor  260 , and an entry guide  270 . 
     The heating roller  210  is, for example, a cylindrical metal core made of metal such as aluminum or a cylindrical metal core whose outer peripheral surface is coated with a fluorine resin or the like, and an outer diameter thereof is, for example, about 50 to 60 [mm]. The heating roller  210  includes a heater (for example, a halogen heater)  211  as a heating unit that heats the fixing belt  220 . 
     The fixing belt  220  is obtained by, for example, coating an outer peripheral surface of a polyimide (PI) substrate having a thickness of 70 [μm] with an elastic layer, and further coating a surface layer with a heat-resistant resin. As the elastic layer, for example, heat-resistant silicone rubber (hardness JIS-A30 [°]) having a thickness of 200 [μm] can be used. As the heat-resistant resin, for example, a PFA (perfluoroalkoxy) tube having a thickness of 50 [μm] can be used. The fixing belt  220  is an endless belt, and is stretched between the heating roller  210  and the fixing roller  230  with a predetermined belt tension (for example, 250 [N]). An outer diameter of the fixing belt  220  is, for example, 120 [mm]. 
     The fixing belt  220  comes into contact with paper on which a toner image has been formed and heats the paper at a fixing temperature. Here, the fixing temperature is a temperature (for example, 160 to 200 [° C.]) at which it is possible to supply the amount of heat necessary for melting a toner on the paper, and varies depending on a type of paper or the like on which an image is to be formed. Note that a surface temperature of the fixing belt  220  is detected by the non-contact temperature sensor  201 , and the heating of the heater  211  is controlled by the control unit  500  to be maintained at a predetermined set temperature. 
     The fixing roller  230  functions as a fixing member, and includes, from the inside, a metal core  231  made of cylindrical metal, an elastic layer  232  made of a material such as silicone rubber or foamed silicone rubber and formed on the surface of the metal core  231 , and a releasing layer (not illustrated) such as a fluororesin. An outer diameter of the fixing roller  230  is, for example, about 65 to 75 [mm], and preferably about 70 [mm]. A thickness of the elastic layer  232  may be, for example, about 15 to 25 [mm], preferably about 20 [mm]. An axial length of the fixing roller  230  is a length that sufficiently corresponds to the maximum paper width (for example, 300 [mm]) that can be conveyed. 
     The fixing roller  230  is rotationally driven (moved) in a direction R of  FIG. 3 , that is, clockwise, for example, when power is transmitted from the first drive motor  250 . A circumferential velocity (first velocity) of the fixing roller  230  can be set to (for example, about 300 to 700 [mm/s]). The fixing belt  220  rotates (moves) following the rotation of the fixing roller  230 . Accordingly, the fixing roller  230  is indirectly heated by the heater  211  via the fixing belt  220 . 
     The first drive motor  250  may be a brushless motor, for example. In this case, the control unit  500  controls a current flowing through a winding of each phase by controlling the magnitude and direction of a voltage applied to each of the U, V, and W phases of the first drive motor  250 . For the current control of the first drive motor  250 , for example, an inverter circuit and a PWM (Pulse Width Modulation) circuit can be used. 
     The pressure roller  240  functions as a pressure member, and includes, from the inside, a metal core  241  made of cylindrical metal, an elastic layer  242  made of a material such as silicone rubber or foamed silicone rubber and formed on the surface of the metal core  241 , and a releasing layer (not illustrated). In the present embodiment, an outer diameter of the pressure roller  240  is set to be the same as the outer diameter of the fixing roller  230  at the room temperature before heating by the heater  211 . In addition, an axial length of the pressure roller  240  and the releasing layer can be configured to be the same as those of the fixing roller  230 . On the other hand, in the present embodiment, a thickness of the elastic layer  242  of the pressure roller  240  is smaller than that of the elastic layer  232  of the fixing roller  230 , and can be formed to be, for example, about 2 to 5 [mm], and preferably about 3 [mm]. 
     In this manner, the thickness of the elastic layer  232  of the fixing roller  230  is formed to be larger than the thickness of the elastic layer  242  of the pressure roller  240 , and thus, the fixing roller  230  becomes relatively softer than the pressure roller  240  at a fixing nip portion N. When the pressure roller  240  is pressed against the fixing roller  230  via the fixing belt  220  with a predetermined fixing load, surface shapes of the fixing roller  230  and the fixing belt  220  are concave while the relatively hard pressure roller  240  exhibits a small deformation amount so that a surface shape thereof is convex. As a result, the surface of the fixing belt  220  easily comes into close contact with minute irregularities of paper, and the paper is discharged along a curvature of the pressure roller  240  due to the convex shape of the surface of the pressure roller  240 , and thus, there is an advantage that the paper is easily separated from the fixing belt  220 . 
     In addition, the pressure roller  240  can be rotationally driven (moved) in a direction opposite to the direction R, that is, counterclockwise when power is transferred from the second drive motor  260 . A circumferential velocity (second velocity) of the pressure roller  240  can be set to (for example, about 300 to 700 [mm/s]). The second drive motor  260  may be, for example, a brushless motor similarly to the first drive motor  250 . The fixing belt  220  and the pressure roller  240  form the fixing nip portion N. 
     The entry guide  270  guides entry of the paper conveyed from the image forming unit  130  into the fixing nip portion N. The entry guide  270  is arranged toward the fixing nip portion N from a conveyance path from the image forming unit  130  to the fixing unit  200 . 
     &lt;Gloss Control&gt; 
     A method for controlling the gloss of an image to be fixed on paper by the fixing unit  200  will be described with reference to  FIGS. 4 to 6 .  FIG. 4  is a conceptual view for describing a cross-sectional shape of a toner image and a cross-sectional shape of a dot after normal fixing; In addition,  FIG. 5  is a schematic view illustrating a relationship between the magnitude of a shearing force applied to a toner particle of the toner image and the deformation of the toner particle. Note that a shape of the toner particle in  FIG. 5  is conceptually illustrated when the toner particle is viewed from above. In addition,  FIG. 6  is a graph illustrating a relationship between the shearing force applied to the toner particle of the toner image, a deformation amount and glossiness of the toner particle. In  FIG. 6 , the horizontal axis of the graph represents the shearing force, and the vertical axis represents a deformation amount or glossiness of a toner. 
     As illustrated in  FIG. 4 , a toner image  10  includes a large number of toner particles  12  stacked on paper  11  in a plurality of layers. These toner particles  12  are deformed so as to be stretched by being heated and pressurized when being fixed in the fixing unit  200 , and are fused to the surface of the paper  11 . As a result, a dot image  14  is constituted by a large number of dots  13  fused to the paper  11 . The dots  13  after fixing correspond to dots of the toner image  10  before fixing, that is, a stacked body (hereinafter simply referred to as a “toner layer  15 ”) of the toner particles  12 . 
     In the present embodiment, the gloss of an image to be fixed on the paper  11  is controlled by adjusting the magnitude of the shearing force to be applied to the toner layer  15 . More specifically, the control unit  500  controls the first drive motor  250  and the second drive motor  260  to set a velocity difference between the circumferential velocity of the fixing belt  220  and the circumferential velocity of the pressure roller  240 , thereby applying the shearing force to the toner layer  15 . 
     As illustrated in (A) in  FIG. 5 , normally, an unfixed toner particle of the toner image have a spherical shape, and a diameter thereof is, for example, about 6 [μm]. 
     The toner image is fixed on the paper  11  at the fixing nip portion N of the fixing unit  200 . During normal fixing, the control unit  500  drives the second drive motor  260  without driving the first drive motor  250  and drives the fixing roller  230  by the rotation of the pressure roller  240 . Therefore, the circumferential velocity of the fixing belt  220  becomes slightly slower than the circumferential velocity of the pressure roller  240 , and a small shearing force is generated between the surface of the fixing belt  220  and the surface of the pressure roller  240 . As a result, the small shearing force is applied to the toner layer  15  of the toner image  10 , the toner particles  12  contained in the toner layer  15  are deformed and fixed so as to be slightly stretched in a direction of the shearing force indicated by an arrow (see (C) of  FIG. 5 ). In this case, the toner particle  12  after fixing have an elliptical shape, and the major diameter thereof can be about 19 [μm] and the minor diameter thereof can be about 15 [μm]. 
     As illustrated in  FIG. 6 , the circumferential velocity of the pressure roller  240  is faster than that in the case of normal fixing, and a large shear is generated between the surface of the fixing belt  220  and the surface of the pressure roller  240  when the circumferential velocity of the fixing belt  220  further becomes relatively slower than the circumferential velocity of the pressure roller  240 . Therefore, since the shearing force applied to the toner layer  15  of the toner image  10  becomes large, the deformation amount of the toner particles  12  increases, and the toner particles  12  are stretched so that the glossiness of the toner image increases. In this specification, making the circumferential velocity of the fixing belt  220  slower than the circumferential velocity of the pressure roller  240  is referred to as “brake”. That is, when the brake is applied, a shearing force is generated, and the glossiness of the dot image  14  becomes high (see (D) in  FIG. 5 ). 
     On the other hand, as the circumferential velocity of the fixing belt  220  is made faster than the circumferential velocity of the pressure roller  240  (also referred to as “assist” in this specification), the shearing force applied to the toner layer  15  of the toner image  10  decreases or no shearing force is applied. In such a case, the deformation amount of the toner particle  12  becomes small, and the glossiness of the dot image  14  becomes low. When no shearing force is applied, the toner particles are uniformly stretched and fixed in any direction, the toner particle  12  after fixing has a circular shape (see (B) in  FIG. 5 ), and a diameter thereof can be about 15 [μm]. 
     Furthermore, when the circumferential velocity of the fixing belt  220  is made much faster than the circumferential velocity of the pressure roller  240  (also referred to as “over-assist” in this specification), the shearing force is applied to the toner layer  15  in the opposite direction to the brake and the toner particles are deformed in the opposite direction, and thus, the glossiness of the dot image  14  becomes high (see (D) in  FIG. 5 ). The major diameter of the toner particle after fixing can be about 26 [μm], and the minor diameter thereof can be about 15 [μm]. 
     &lt;Relationship Between Gloss Control and Image Quality&gt; 
     The inventor of the present invention has conducted experiments on the normal fixing and the fixing in the case of gloss control and evaluated experimental results, and as a result, has found that there is a relationship between the gloss control and the image quality of a dot image after fixing. 
       FIG. 7A  is a photograph of a top view of a dot image on paper in the case of normal fixing, and  FIG. 7B  is a photograph of a top view of a dot image on paper when a toner layer has been fixed by applying a shearing force (brake). Note that  FIGS. 7A and 7B  are all photographs captured at a magnification of about 500 times using a laser microscope (manufactured by Keyence Corporation). In addition,  FIG. 8  is a graph which illustrates a result of measuring a height of a dot with respect to a distance of the dot in a paper passage direction in the case where no shearing force is applied to the toner layer and the case where the shearing force (brake) is applied. In  FIG. 8 , the horizontal axis of the graph represents the distance [μm] in the paper passage direction within the dot, and the vertical axis represents the dot height [μm]. 
     As illustrated in  FIG. 7A , there is no shearing force applied to the toner layer  15  or a small shearing force is applied in the case of normal fixing, and thus, the density of each portion in each of dots  22  of a dot image  20  on paper  21  is approximately the same. 
     On the other hand, when the toner layer is fixed by applying the shearing force (brake) as illustrated in  FIG. 7B , a dark part and a light part can be generated in each portion of each of dots  23  on the paper  21 . In this case, the circumferential velocity of the fixing belt is slower than the circumferential velocity of the pressure roller at the time of fixing, and thus, an upstream density of a dot is low and a downstream density thereof is high. 
     In addition, even in the case where a shearing force (assist) is applied to the toner layer for fixing, a part having a high density and a part having a low density can occur in each portion of each dot on the paper  21  although not illustrated. In this case, the circumferential velocity of the fixing belt is faster than the circumferential velocity of the pressure roller at the time of fixing, and thus, an upstream density of a dot is high and a downstream density thereof is low. 
     When the normal fixing is performed as illustrated in  FIG. 8 , a dot height (gray solid line  31 ) is about 13 [μm] and is almost constant in a range where the distance in the paper passage direction is about 20 to 70 [μm]. In addition, the dot height decreases from the center to an upstream end as substantially indicated by an inclined line  32  in a range where the distance in the paper passage direction is about 70 to 100 [μm]. 
     On the other hand, when the toner layer is fixed by applying the shearing force (brake), a dot height (black solid line  33 ) is approximately 13 [μm] and is almost constant in a range where the distance in the paper passage direction is about 20 to 40 [μm]. In addition, the dot height decreases from the downstream side to an upstream end as substantially indicated by an inclined line  34  in a range where the distance in the paper passage direction is about 40 to 100 [μm]. 
     In this manner, when the normal fixing is performed, a dot thickness has many flat portions from the upstream side to the downstream side, and the density thereof is maintained substantially uniform. On the other hand, when the toner layer is fixed by applying the shearing force (brake), a dot thickness is thin on the upstream side and thick on the downstream side so that unevenness occurs in the density since the dot thickness is different between the upstream side and the downstream side. 
     In addition, when the toner layer is fixed by applying a shearing force (over-assist), a dot thickness is thick on the upstream side and thin on the downstream side although not illustrated, which is opposite to the case of brake. Since the dot thickness is different between the upstream side and the downstream side of the dot even in the case of over-assist, unevenness occurs in the density. 
     &lt;Density Unevenness&gt; 
       FIG. 9  is a schematic view illustrating a relationship between a dot image pattern and density unevenness, and  FIG. 10  is a graph illustrating a relationship between a dot slope and a velocity difference [%]. The horizontal axis of  FIG. 10  represents the velocity difference [%] between the circumferential velocity of the fixing belt and the circumferential velocity of the pressure roller, and the vertical axis represents the dot slope. 
     Whether density unevenness is visually conspicuous depends on an individual difference on the image-viewing side, but it is possible to set a determination criterion to some extent on how much density unevenness is conspicuous based on determination results of density unevenness by many users and past determination results. 
     For example, whether density unevenness is visually conspicuous depends on a dot image pattern. In  FIG. 9 , where an image density is high (for example, 95 to 100%), that is, an image pattern is a solid image having a large area, dots are formed so as to be dense in a portion and thus, density unevenness is often relatively inconspicuous (unnoticeable). 
     On the other hand, when a dot image is a halftone with a halftone dot pattern, dots becomes sparse as compared to the solid image, and thus, density unevenness is sometimes conspicuous depending on an image density. For example, when the image density is 25 to 45 [%], the deterioration of image quality caused by the density unevenness is particularly conspicuous. 
     In addition, a change of a cross-sectional shape of a dot after fixing also becomes large as the velocity difference between the circumferential velocity of the fixing belt and the circumferential velocity of the pressure roller (hereinafter also simply referred to as the “velocity difference”) is increased and a shearing force applied to a toner layer by the brake or over-assist is increased. Therefore, the density unevenness becomes more conspicuous. The degree of deformation of the cross-sectional shape of the dot is defined by a slope S=B/A (see  FIG. 8 ). The slope S is equal to the dot slope (inclined line  34 ). As a result of the investigation on the density unevenness, a result that the density unevenness was conspicuous was obtained when the slope S was 0.1 or less as a determination criterion. Note that the slope S can be acquired by a known method. For example, the slope S can be acquired by calculating an approximate straight line based on a distribution of data of the black solid line  33  illustrated in  FIG. 8 . 
     As illustrated in  FIG. 10 , the dot slope S is 0.1 or less when the velocity difference is 1% or more. 
     &lt;Measurement of Velocity Difference&gt; 
       FIG. 11A  is a schematic view for describing a marking  290  attached on the fixing belt  220 , and  FIG. 11B  is a schematic view for describing a marking trace attached to paper after fixing. 
     As illustrated in  FIG. 11A , the marking  290  is provided on the fixing belt  220  to fix an image longer than a circumferential length of the fixing belt  220 . As a result, traces  42  and  43  of the marking  290  attached to the fixing belt  220  appear on the fixed image on paper  41  as illustrated in  FIG. 11B . A distance between markings on this fixed image is measured and is defined as L 1 . The distance L 1  corresponds to a distance traveled by the fixing belt  220  in one round. 
     As described above, the surface shapes of the fixing roller  230  and the fixing belt  220  are concave since the surface of the pressure roller  240  is relatively harder than the surfaces of the fixing roller  230  and the fixing belt  220 . For this reason, the fixing belt  220  travels longer at the fixing nip portion N by the amount of the concave shape, and as a result, the circumferential velocity of the fixing belt  220  becomes faster at the fixing nip portion N. When the circumferential length of the fixing belt  220  is L 2 , a velocity difference ΔV [%] is expressed by ΔV=(L 1 −L 2 )/L 2 . 
     &lt;Configuration for Suppression of Image Quality Deterioration&gt; 
       FIGS. 12A and 12B  are conceptual views for describing a cross-sectional shape of a toner image  10  formed by the image forming unit  130  of the present embodiment and a cross-sectional shape of the dot  13  fixed by applying a shearing force to the toner layer  15 .  FIG. 12A  illustrates the case where fixing is performed with the brake, and  FIG. 12B  is the case where fixing is performed with the assist. In addition,  FIGS. 13A and 13B  are conceptual views for describing an example and another example, respectively, in which a laser output of the image forming unit  130  is changed as means for changing the density of the toner image  10 . In addition,  FIG. 14  is a conceptual view for describing a change amount in the density change of the toner image  10 . 
     In the present embodiment, the toner image  10  is formed so as to interpolate, in advance, for the deformation of the cross-sectional shape of the toner layer  15  caused by the shearing force applied to the toner layer  15  of the toner image  10  accompanying the gloss control. When the circumferential velocity of the fixing belt  220  and the circumferential velocity of the pressure roller  240  are different at the fixing nip portion N, the control unit  500  determines the amount of toner to be attached to the paper  11  such that the distribution of the toner amount is different in each dot before fixing. More specifically, the control is performed as follows. 
     When fixing is performed with the brake as illustrated in  FIG. 12A , the control unit  500 , for each of the dots  13  of the dot image  14  after fixing, sets the distribution of the toner amount in each dot before fixing to be different between the upstream side and the downstream side such that the density on the upstream side becomes the same as the density on the downstream side. 
     As described above, when the number of layers of toner particles stacked in the toner image is formed to be uniform in the dot and fixing is performed with the brake, the dot thickness in the dot image after fixing is thicker on the downstream side and gradually becomes thinner toward the upstream end. Therefore, the control unit  500  controls the amount of toner to be attached to the paper  11  by the image forming unit  130  such that the number of layers of the toner particles  12  is gradually increased from the downstream side to the upstream side of the toner layer  15  at the time of forming the toner image  10  in the case of fixing with the brake. As a result, the toner amount on the upstream side of the dot of the toner image  10  (that is, the toner layer  15 ) becomes larger than the toner amount on the downstream side, and the density on the upstream side of each of the dots  13  of the dot image  14  becomes substantially the same as the density on the downstream side after fixing. As a result, the density from the upstream side to the downstream side of each of the dots  13  of the dot image  14  becomes substantially constant. 
     In addition, the control unit  500  controls the amount of toner attached to the paper  11  such that the downstream density of each of the dots  13  of the dot image  14  after fixing is substantially the same as the upstream density even in the case of fixing with the over-assist as illustrated in  FIG. 12B . 
     As described above, when the number of layers of toner particles stacked in the toner image is formed to be uniform in the dot and fixing is performed with the over-assist, the dot thickness in the dot image  14  after fixing is thicker on the upstream side and gradually becomes thinner toward the downstream end. Therefore, the control unit  500  controls the amount of toner to be attached to the paper  11  by the image forming unit  130  such that the number of layers of the toner particles  12  is gradually increased from the upstream side to the downstream side of the dot at the time of forming the toner image. As a result, the toner amount on the downstream side of the dot of the toner image  10  (that is, the toner layer  15 ) becomes larger than the toner amount on the upstream side, and the density on the downstream side of each of the dots  13  of the dot image  14  becomes substantially the same as the density on the upstream side after fixing. As a result, the density from the upstream side to the downstream side of each of the dots  13  of the dot image  14  becomes substantially constant. 
     The toner amount is changed by adjusting the laser output of the optical writing unit  3  of the image forming unit  130 , for example. The control unit  500  controls an exposure condition of the optical writing unit  3 , for example, controls the magnitude of the laser output to determine the amount of toner to be attached to the paper  11 . 
     As illustrated in  FIG. 13A , the control unit  500  outputs a control signal to the image forming unit  130  such that the laser output increases from the downstream side toward the upstream side so as to increase the number of layers from the downstream side to the upstream side of the dot when fixing is performed with the brake. For example, when a normal laser output is set to ten regarding the magnitude of the laser output, the control unit  500  can increase the laser output on the upstream side stepwise from ten with respect to ten of the downstream side and set an upstream end  16  to a maximum value of sixteen. 
     In addition, for example, the control unit  500  can also set a downstream end  17  to eight, which is smaller than that at the time of normal fixing, regarding the magnitude of the laser output, and increase the laser output on the upstream side stepwise from eight and set the upstream end  16  to a maximum value of sixteen as illustrated in  FIG. 13B . 
     In addition, the control unit  500  interpolates the deformation of the cross-sectional shape of each dot of the toner image  10  caused by the velocity difference between the circumferential velocity of the fixing belt  220  and the circumferential velocity of the pressure roller  240 , and increases the amount of toner to be attached to the paper  11  such that the density from the upstream side to the downstream side of each dot after fixing is constant. Specifically, when a slope of a toner change amount is defined as T=b/a as illustrated in  FIG. 14 , the control unit  500  adjusts the magnitude of the laser output such that the slope T and the dot slope S are equal. 
     In addition, the deterioration in image quality caused by the density unevenness is particularly conspicuous when the image density is 25 to 45 [%] as described above, and thus, the control unit  500  may be configured to adjust the magnitude of the laser output when the image density of the dot image is 25 to 45 [%]. 
     In addition, the density unevenness is conspicuous when the dot slope S is 0.1 or less as described above. In addition, the dot slope S is 0.1 or less when the velocity difference is 1% or more. Therefore, the control unit  500  may be configured to control the amount of toner attached to the paper  11  when the velocity difference is 1 [%] or more. 
     Furthermore, the change in the cross-sectional shape of the dots  13  after fixing also increases as the velocity difference increases and the shearing force applied to the toner by the brake or over-assist increases as described above, the density unevenness becomes more conspicuous. Therefore, the control unit  500  may be configured to control the amount of toner attached to the paper  11  in response to the velocity difference. For example, the toner change amount is increased when the velocity difference is large, and the toner change amount is decreased when the velocity is small. 
     The image forming apparatus  100  having the fixing unit  200  and the fixing unit  200  of the present embodiment described above has the following effects. 
     When the circumferential velocity of the fixing belt  220  and the circumferential velocity of the pressure roller  240  are different at the fixing nip portion N, the control unit  500  determines the amount of toner to be attached to the paper  11  such that the distribution of the toner amount is different in each dot before fixing. Therefore, the density unevenness can be prevented or suppressed even when the shearing force is applied between the toner image  10  on the paper  11  and the fixing belt  220  accompanying the gloss control. 
     As described above, the fixing unit  200  and the image forming apparatus  100  have been described in the embodiment. However, it is a matter of course that the present invention can be appropriately added, modified, and omitted by those skilled in the art within the scope of the technical idea. 
     For example, the case where the fixing unit  200  includes the fixing belt  220  has been illustrated in the above-described example, the present invention is not limited to such a case, and the present invention can be applied to a configuration in which the fixing unit  200  does not include the fixing belt  220 . In such a case, the fixing roller  230  functions as a fixing member. 
     In the above example, the case where the control unit  500  controls the toner attachment amount has been described, but the present invention is not limited to such a case, and the fixing unit may include a control unit, and the control unit of the fixing unit may be configured to control the toner attachment amount. 
     Although embodiments of the present invention have been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and not limitation, the scope of the present invention should be interpreted by terms of the appended claims.