Patent Publication Number: US-7899379-B2

Title: Image forming apparatus and toner manufacturing method

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 2007-005555 filed on Jan. 15, 2007, the entire contents of which are incorporated herein by reference. 
     BACKGROUND 
     1. Field of the Invention 
     The present invention relates to an image forming apparatus and a toner manufacturing method. 
     2. Description of the Related Art 
     In an image forming apparatus, among the cost of copying and printing, the rate occupied by toner is very high. Therefore, to reduce the cost, it is desirable to reduce the toner quantity adhered to a sheet to form an image. Therefore, conventionally, there is an image forming apparatus using toner having a toner save function for reducing the toner quantity to be consumed. 
     As a toner quantity reducing method when using the toner save function, for example, as described in Japanese Patent Application Publication No. 11-308450, there are a method for reducing the toner consumption quantity by reducing the number of printed dots by a method for thinning out the main scanning lines, that is, performing a thinning process every several dots in the overall image area and a method for reducing the toner quantity of each pixel available. 
     However, the toner save function by the conventional thinning process does not take the image quality into account. Among the toner components, the one contributing most to the image density is a pigment. When the toner consumption quantity is reduced, the pigment quantity per image unit area is reduced, so that the image density is lowered, thus a problem arises that a satisfactory image cannot be obtained. 
     SUMMARY 
     An object of the present invention is to provide an image forming apparatus and a toner manufacturing method for maintaining a satisfactory image even if reducing the toner quantity in the image formation. 
     According to the embodiments of the present invention, there are provided an image forming apparatus comprising toner with a coloring agent including magnetic powder as a nucleus arranged on a surface of one side of each toner particle; a toner image forming medium on which a toner image is formed using the toner; and a magnetic substance arranged at an opposite position to the toner image formed on the toner image forming medium. 
     Further, according to the embodiments of the present invention, there are provided a toner manufacturing method comprising forming a monomer composition for dispersing a mixture of additives such as a polymeric monomer, coloring agents including magnetic powder as a nucleus, a polymerization initiator, a cross-linker and a charging control agent; stirring the monomer composition in a water tank including a stabilization agent, thereby obtaining toner particles; and giving a magnetic field to the water tank and concentrating the coloring agents on one side of each of the toner particles. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a schematic cross sectional view showing the main unit of the image forming apparatus which is an embodiment of the present invention; 
         FIG. 2  is a schematic view showing a toner particle with the coloring agents used in Embodiment 1 of the present invention arranged on one side thereof; 
         FIG. 3  is a schematic view showing the coloring agent; 
         FIG. 4  is a schematic view for explaining the process of aligning the directions of toner particles at the development step; 
         FIG. 5  is a schematic view for explaining the process of aligning the directions of toner particles at the transfer step; 
         FIG. 6  is a schematic view of the fixing device used in Embodiment 2 of the present invention; 
         FIG. 7  is a drawing for explaining the fixing step of Embodiment 2 of the present invention; and 
         FIG. 8  is a schematic view of a toner particle with the coloring agents used in Embodiment 2 of the present invention arranged overall. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Hereinafter, the embodiments of the present invention will be explained with reference to the accompanying drawings. 
     Embodiment 1 
       FIG. 1  shows, as an image forming apparatus relating to the embodiments of the present invention, a schematic cross sectional view of the color image forming apparatus using the intermediate transfer system. 
     A color image forming apparatus  101  stores, in a frame  109 , a document reading unit  103 , an image forming unit  105 , and a sheet feed unit  107 . The document reading unit  103  irradiates light to a document not drawn which is set on the document table, leads the reflected light from the document to the light receiving element via a plurality of optical members, converts it photo-electrically, thereby outputs image data. 
     Further, the image forming unit  105  outputs an image based on image data read from a document by the document reading unit  103  or image data inputted from an external apparatus not drawn onto a recording medium P. Furthermore, the sheet feed unit  107  feeds the recording medium P to the image forming unit  105 . 
     On the frame  109 , an automatic duplex unit  111  and a manual paper supply unit  113  are attached removably. The automatic duplex unit  111  reverses the recording medium P on one side of which an image is formed by the image forming unit  105  and feeds it again to the image forming unit  105 , thus an image is formed on the residual one side. The manual paper supply unit  113  supplies manually the recording medium P to the image forming unit  105 . 
     Next, the image forming unit  105  will be explained in detail. The image forming unit  105  has a photosensitive drum  115  which is a toner image forming medium having a pipe shaft extending in the longitudinal direction (the depth direction of the drawing) of the color image forming apparatus  101 . Around the photosensitive drum  115 , a main charger  117 , an exposure unit  119 , a black developing device  121 , a revolver  123  which is a color developing device, an intermediate transfer belt  125  which is a transfer medium, and a drum cleaner  127  are installed in the rotational direction (the direction of the arrow in the drawing) of the photosensitive drum  115 . 
     The main charger  117  charges the outer peripheral surface of the photosensitive drum  115  at a predetermined potential. The exposure unit  119  is arranged in the neighborhood of the lower end of the image forming unit  105 , exposes the surface of the photosensitive drum  115  which is charged at the predetermined potential, thereby forms an electrostatic latent image based on the image data. When forming a color image, the exposure unit  119  exposes the surface of the photosensitive drum  115  on the basis of the color-resolved image data and forms an electrostatic latent image of each color. 
     The black developing device  121  is arranged between the photosensitive drum  115  and the exposure unit  119 , that is, opposite to the photosensitive drum  115  from underneath. The black developing device  121  adheres and develops black toner to the electrostatic latent image for black formed on the surface of the photosensitive drum  115  by the exposure unit  119  and forms a black toner image on the surface of the photosensitive drum  115 . 
     The black developing device  121  has a mixer for stirring and feeding toner and a black developing roller  121   b  which is a magnetic roller arranged opposite to it on the surface of the photosensitive drum  115  via a predetermined development gap. The black developing device  121  is installed movably so as to permit the black developing roller  121   b  to make contact with and separate from the surface of the photosensitive drum  115 . Further, to the black developing device  121 , toner is fed from a toner cartridge  121   a  via a feed path not drawn. 
     The revolver  123  is installed in the neighborhood of the photosensitive drum  115  so as to rotate clockwise. The revolver  123  has a yellow developing device  123 Y, a magenta developing device  123 M and a cyan developing device  123 C which have the same structure as that of the black developing device  121 . The developing devices are arranged side by side in the rotational direction of the revolver  123  and are stored removably in the revolver  123 . The color developing devices  123 Y,  123 M and  123 C have a yellow developing roller  123 Ya, a magenta developing roller  123 Ma and a cyan developing roller  123 Ca which are arranged opposite to them on the surface of the photosensitive drum  115  via a predetermined gap. 
     The color developing devices  123 Y,  123 M and  123 C, when the revolver  123  rotates clockwise, are arranged selectively opposite to the surface of the photosensitive drum  115  from the side thereof. 
     The black developing device  121 , since the frequency in use is higher than those of the developing devices of the other colors, is installed separately from the revolver  123  storing the developing devices of the other colors. Therefore, the toner storage capacities of the developing devices and toner cartridges can be made different from those of the developing devices of the other colors, thus the number of times of maintenance such as toner feed can be reduced. 
     The intermediate transfer belt  125  is arranged above the photosensitive drum  115 . The intermediate transfer belt  125  is wound round a drive roller  125   a  having the rotary shaft extending in the longitudinal direction (the depth direction of the drawing) of the color image forming apparatus  101 , a driven roller  125   b , a driven roller  125   c , and a tension roller  125   d  and is stretched by them. The drive roller  125   a  is installed fixedly to the frame  109  above the revolver  123 . The tension roller  125   d , so as to give predetermined tension to the intermediate transfer belt  125 , is pressed from the inside of the intermediate transfer belt  125  toward the outside thereof. 
     Inside the intermediate transfer belt  125 , to permit the intermediate transfer belt  125  to make contact with the surface of the photosensitive drum  115  and transfer a toner image formed on the surface of the photosensitive drum  115  to the intermediate transfer belt  125 , a first transfer roller  131  is installed. The first transfer roller  131  is pressed toward the photosensitive drum  115  so as to press the intermediate transfer belt  125  to the surface of the photosensitive drum  115  at a predetermined pressure. 
     Around the intermediate transfer belt  125 , a belt cleaner  133  and a secondary transfer roller  135  are installed touchably on the belt surface. The belt cleaner  133  is installed on the outer periphery of the drive roller  125   a  via the intermediate transfer belt  125  above the revolver  123 . 
     The secondary transfer roller  135  of this embodiment has an outside diameter of 28 mm and is composed of an epichloro rubber sponge the surface of which is coated with an epichloro rubber tube. The rubber hardness is 25 to 30 degrees and the volume resistance is 10 7 Ω. 
     Further, the secondary transfer roller  135  is installed via the intermediate transfer roller  125  at the position between the driven roller  125   c  and it across a vertical conveying path  137  which will be described later and forms the secondary transfer area at this part. The drum cleaner  127  is arranged in contact with the photosensitive drum  115 . 
     The sheet feed unit  107  has two sheet feed cassettes  107   a  and  107   b . At the right upper ends of the sheet feed cassettes  107   a  and  107   b  shown in the drawing, pickup rollers  141  ( 141   a ,  141   b ) for taking out the recording media P at the uppermost ends stored in the cassettes are installed. At the positions in the neighborhood of the downstream side in the recording medium takeout direction by the pickup rollers  141 , feed rollers  143  and separation rollers  145  are arranged respectively opposite to each other. 
     Further, at the neighboring position of the sheet feed cassettes  107   a  and  107   b  on the right of the drawing, the vertical conveying path  137  extending almost in the perpendicular direction through the secondary transfer area where the intermediate transfer belt  125  and secondary transfer roller  135  make contact with each other is installed. On the vertical conveying path  137 , a plurality of paired conveying rollers  149  for rotating by holding the recording media P are installed. 
     Above the discharge unit of the recording media P in the secondary transfer area, a fixing device  151  for heating, pressurizing, and fixing a toner image transferred onto each of the recording media P is installed. 
     Further, exit rollers  153  for discharging the image-formed recording media P to a sheet receiving tray  155  is installed. 
     Next, the color image forming operation by the color image forming apparatus will be explained. As an initial operation, the black developing device  121  moves down and separates from the surface of the photosensitive drum  115 , and the revolver  123  rotates clockwise, and the yellow developing device  123 Y faces the surface of the photosensitive drum  115 . Further, the belt cleaner  133  rotates counterclockwise around the support shaft thereof and separates from the intermediate transfer belt  125 , and the secondary transfer roller  135  moves in the direction (in the right direction of the drawing) separating from the vertical conveying path  137  and separates from the intermediate transfer belt  125 . 
     And, image data is read from a document not drawn by the document reading unit  103  or image data is inputted from an external apparatus not drawn. Furthermore, the photosensitive drum  115  rotates clockwise and the surface of the photosensitive drum  115  is charged uniformly at a predetermined potential by the main charger  117 . At this time, the intermediate transfer belt  125  rotates counterclockwise at the same speed as the peripheral speed of the photosensitive drum  115 . 
     Firstly, on the basis of the color-resolved yellow image data, the exposure unit  119  operates and forms an electrostatic latent image for yellow on the surface of the photosensitive drum  115 . At this time, the exposure timing is synchronized by detecting a detection mark, not drawn, stuck inside the intermediate transfer belt  125  by a detector not drawn. 
     The electrostatic latent image for yellow formed on the surface of the photosensitive drum  115  by the yellow developing device  123 Y is adhered with yellow toner and is developed, thus a yellow toner image is formed on the surface of the photosensitive drum  115 . The yellow toner image formed on the surface of the photosensitive drum  115  in this way is moved by rotation of the photosensitive drum  115  and passes through the primary transfer area in contact with the intermediate transfer belt  125 . 
     At this time, a bias voltage with reverse polarity to the charging polarity of each toner particle is given to the primary transfer roller  131  and the yellow toner image on the surface of the photosensitive drum  115  is transferred onto the intermediate transfer belt  125 . 
     After the yellow toner image is transferred onto the intermediate transfer belt  125 , the yellow toner remaining on the surface of the photosensitive drum  115  without being transferred is removed by the drum cleaner  127 . At this time, the residual electric charge on the surface of the photosensitive drum  115  is also neutralized simultaneously. 
     And, to set up for forming the next electrostatic latent image for magenta on the photosensitive drum  115 , the surface of the photosensitive drum  115  is charged uniformly by the main charger  117 , and the revolver  123  is rotated, and the magenta developing device  123 M faces the surface of the photosensitive drum  115 . 
     In this state, a series of processes aforementioned, that is, exposure, development, and primary transfer onto the intermediate transfer belt  125  are executed, and a magenta image is superimposed and transferred onto the yellow toner image on the intermediate transfer belt  125 . 
     After a cyan toner image is transferred similarly, to the position where the developing devices  123 Y,  123 M and  123 C do not face the surface of the photosensitive drum  115 , the revolver  123  rotates and the black developing device  121  moves up in place and faces the surface of the photosensitive drum  115 . In this state, the processes similar to the aforementioned processes are executed, and a black toner image is superimposed on the yellow toner image, magenta toner image, and cyan toner image and is transferred onto the intermediate transfer belt  125 . 
     When the toner images of all the colors are superimposed on the intermediate transfer belt  125  in this way, the secondary transfer roller  135  moves toward the driven roller  125   c  and makes contact with the intermediate transfer belt  125 . Further, the belt cleaner  133  also makes contact with the intermediate transfer belt  125 . The toner images of all the colors superimposed on the intermediate transfer belt  125  in this state rotate due to rotation of the intermediate transfer belt  125  and passes through the secondary transfer area where the intermediate transfer belt  125  and the secondary transfer roller  135  are in contact with each other. 
     At this time, the recording medium P taken out from the sheet feed cassette  107   a  or  107   b  by the pickup roller  141   a  or  141   b  is conveyed upward on the vertical conveying path  137  by the conveying roller  149  and is sent into the secondary transfer area at predetermined timing. 
     And, via the secondary transfer roller  135  impressed with a bias voltage having reverse polarity to that of the toner image of each color by the power source which will be described later, the toner images of all the colors on the intermediate transfer belt  125  are transferred in a batch onto the recording medium P. After the toner images are transferred to the recording medium P, the toner remaining on the intermediate transfer belt  125  is removed by the belt cleaner  133 . The recording medium P on which the toner images of all the colors are transferred together is heated and pressurized by the fixing device  151 , and the toner images of all the colors are fixed on the recording medium P, thus a color image is formed. The recording medium P on which the color image is formed is discharged onto the sheet receiving tray  155  via the exit rollers  153  installed on the downstream side of the fixing device  151 . 
     The toner used in this embodiment will be explained below. As shown in  FIG. 2 , a toner particle  201  used in this embodiment has a particle diameter which is a volume average particle diameter of 6 μm and includes coloring agents  203  arranged on the surface of the complementary hemisphere. Here, as the coloring agents  203 , for example, in the case of black toner, magnetic powder  301  itself can be used or magnetic power with carbon black adhered on the surface of the magnetic powder  301  can be used. In the case of color toner, as the coloring agents  203 , as shown in  FIG. 3 , magnetic powder  301  with a pigment  303  of each color adhered on the surface of the magnetic powder  301  is used. 
     As the magnetic powder  301 , for example, magnetic powder having magnetic coercive force of 10 to 1,800 Oe (0.8 to 143.2 kA/m), saturation magnetization of 50 to 130 emu/g (50 to 130 Am 2 ), and residual magnetization of 1 to 65 emu/g (1 to 65 Am 2 ) is used. Further, the volume average particle diameter is suitably 0.05 to 0.2 μm or so. Further, as shown in  FIG. 3 , the coloring agents  203  have a particle diameter of the order of submicron which is similar to or slightly larger than that of the magnetic powder  301 . 
     As the pigment  303 , a pigment for coloring yellow, magenta, and cyan is used. As a pigment for yellow, a monoazo pigment, a disazo pigment, a condensation azo-pigment, and an isoindolin pigment can be used. As a magenta pigment, a quinacridone pigment, an azo-pigment, a condensation azo-pigment, and a perylene pigment can be used. As a cyan pigment, a phthalocyanine pigment can be used. 
     Next, a method for manufacturing toner composed of the magnetic powder  301  of this embodiment arranged on the surface of a complementary hemisphere will be explained. As a manufacturing method, the suspension polymerization method is suitable. In the suspension polymerization method, toner is obtained by stirring a monomer composition for dispersing a mixture of additives such a polymeric monomer, the coloring agents  203  composed of magnetic powder  301  as a nucleus, moreover, a polymerization initiator, a cross-linker, and a charging control agent in a water tank including a stabilization agent, thereby producing particles, and polymerizing them to a desired particle size. 
     The coloring agents  203  composed of the magnetic powder  301  as a nucleus has high specific gravity, so that the gravity direction in the particles is concentrated on one side ( FIG. 2 ). When the offset is insufficient, by formation of a magnetic field during production of particles, the offset of the coloring agents  203  in the direction of magnetic force can be assisted. 
     Next, the image forming process when using the toner particles  201  aforementioned will be explained. To uniform the toner directions at the developing step, the developing rollers  121   b ,  123 Ya,  123 Ma and  123 Ca are acceptably magnet rollers. 
     When the developing rollers  121   b ,  123 Ya,  123 Ma and  123 Ca are formed by magnet rollers, a magnetic field is formed between the photosensitive drum  115  and the developing rollers  121   b ,  123 Ya,  123 Ma and  123 Ca, thus at time of development, the sides of the toner particles  201  where the coloring agents  203  are arranged are uniformed in the state that they face the developing rollers  121   b ,  123 Ya,  123 Ma and  123 Ca. 
     Therefore, as shown in  FIG. 4 , in the toner particles  201  on the photosensitive drum  115 , the coloring agents  203  are adhered in the state that they are all directed toward the surface. And, the toner particles  201  adhered to the photosensitive drum  115  are transferred to the intermediate transfer belt  125 . The toner particles  201  transferred to the intermediate transfer belt  125 , as shown in  FIG. 4 , are in the state that the coloring agents  203  are directed toward the intermediate transfer belt  125 . 
     In this state, the toner particles  201  are moved by rotation of the intermediate transfer belt  125  and when they reach the secondary transfer area, the toner particles  201  are transferred to the recording medium P. The toner particles  201  transferred to the recording medium P are in the state that the coloring agents  203  are arranged side by side on the surface of the recording medium P. 
     As mentioned above, when forming an image so that the coloring agents are arranged on the surface of the complementary hemisphere of each toner particle and the coloring agents are arranged side by side on the surface of the recording medium P, even if the adhesion quantity of toner is little, an image at high image density is obtained. 
     Further, in the explanation aforementioned, the directions of the toner particles are uniformed at the developing step, though when a proper magnetic field is applied within the range that the toner particles do not move, it is possible to rotate the toner particles and uniform the directions thereof. 
     For example, at the transfer step, a means for aligning the directions of toner particles will be explained. As shown in  FIG. 5 , a magnetic member  501  is arranged inside the driven roller  125   c . By doing this, even if toner particles are transferred onto the intermediate transfer belt  125  in the state that the directions of the coloring agents  203  are not uniform, by the magnetic member  501  inside the driven roller  125   c , when they are transferred from the intermediate transfer belt  125  to the recording medium P, the sides of the toner particles  210  where the coloring agents  203  are arranged by the magnetic field are arranged side by side on the surface of the recording medium P. 
     Embodiment 2 
     In the embodiment aforementioned, the toner particles  201  composed of the coloring agents  203  arranged on the surface of the complementary hemisphere are used, while in this embodiment, as shown in  FIG. 8 , the case that toner particles  801  composed of the coloring agents  203  arranged free of offset on the surface of each toner particle will be explained. In the toner particles  801  used in this embodiment, the coloring agents  203  arranged on the surface are smaller in the amount than the conventional toner particles. 
       FIG. 6  is a drawing showing the schematic constitution of a fixing device  151 ′ of Embodiment 2. In the fixing device  151 ′ of this embodiment, a fixing belt  601 , a fixing roller  603 , a tungsten halogen lamp  605 , a magnetic member  607 , a cooling mechanism  609 , and a backup roller  611  are installed. The tungsten halogen lamp  605  and magnetic member  607  are installed inside the fixing roller  03  and when the tungsten halogen lamp  605  is turned on, heat is given to the toner particles  801  on the recording medium P via the fixing belt  601 . Further, by the magnetic member  607 , a magnetic field is generated in the fixing nip section. 
     The fixing step of this fixing device  151 ′ will be explained below. When the recording medium P on the surface of which the toner particles  801  are transferred is conveyed to the fixing nip section of the fixing device  151 ′, the integrating resin composing the toner particles  801  is melted by heat temporarily to a liquid. 
     As shown in  FIG. 7 , in the resin melted to a liquid, there are the coloring agents  203  including the magnetic powder  301  as a nucleus and the coloring agents  203  are attracted toward the fixing roller  603  in the resin melted to a liquid by the magnetic member  607  installed inside the fixing roller  603 . The recording medium P coming out from the fixing nip is conveyed in contact with the fixing belt  601  and the toner particles  801  are cooled and hardened by the cooling mechanism  609 . 
     When forming an image using the toner particles  801  in which the coloring agents  203  in a slightly small amount than general toner are arranged allover the surfaces of the toner particles  801  and the fixing device aforementioned, the coloring agents  203  for deciding the image density can be concentrated on the surface of the recording medium P, thus an image at a high image density can be obtained using very few toner particles  801 .