Patent Publication Number: US-7218877-B2

Title: Charging device, and process cartridge and image forming apparatus including the charging device using the same

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of U.S. patent application Ser. No. 10/843,574, filed May 12, 2004 now U.S. Pat. No. 7,062,194, which is based upon and claims the benefit of priority from Japanese Patent Applications Nos. 2003-132990 and 2004-024958, filed on May 12, 2003 and on Feb. 2, 2004, respectively, incorporated herein by reference. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   The present invention relates to a charging device which charges an image bearing member with a charging roller in electrophotographic image forming apparatus and which has a cleaner cleaning the charging roller. In addition, the present invention also relates to an image forming apparatus such as copiers and printers which use the cleaning device and a process cartridge using the charging device. 
   2. Discussion of the Background 
   In conventional electrophotographic image forming apparatus, an image is typically formed by the following method:
     (1) an image bearing member such as photoreceptors is charged with a charger to apply a charge having a predetermined polarity to the image bearing member (i.e., charging process);   (2) the image bearing member is exposed to imagewise light to form a latent electrostatic image on the image bearing member (i.e., light irradiation process);   (3) the latent electrostatic image is developed with a toner having a charge with the same polarity as that of the latent electrostatic image to form a toner image (i.e., developing process);   (4) the toner image is transferred to a receiving material such as papers (i.e., transferring process); and   (5) the toner image is fixed on the receiving material upon application of heat and pressure thereto to form a hard copy (i.e., fixing process).   

   Even after the transfer process, a small amount of toner particles remains on the surface of the image bearing member. Therefore, the surface of the image bearing member is typically cleaned by a cleaner, such as cleaning blades and cleaning brushes, before the next charging process to remove the residual toner particles from the surface of the image bearing member. 
   Recently, contact charging methods in which a voltage is applied to an image bearing member by an electroconductive charging roller which is contacted with the image bearing member or short-range charging methods in which a voltage is applied to an image bearing member by an electroconductive charging roller which is set in a close vicinity of an image bearing member are typically used for the charging process. This is because these charging methods have advantages such that the amount of ozone generated due to discharging caused by the charger can be restrained and the power consumption of the charger can be reduced. 
   However, when residual toner particles are insufficiently removed, a problem occurs in that, when the remaining toner particles contact with or are close to the charging roller, the remaining toner particles may adhere thereto. This is because the remaining toner particles typically include toner particles which have a charge with a polarity opposite to the polarity of the potential of the charging roller, and the reversely-charged toner particles are attracted by the charging roller, resulting in adhesion of the toner particles to the surface of the charging roller. In addition, dust such as paper dust generated by receiving papers, which has a charge with a polarity opposite to that of the potential of the charging roller can be also adhered to the charging roller. 
   Recently a need exists for an electrophotographic image forming apparatus capable of producing high quality and high definition images. Therefore, a spherical toner having a relatively small particle diameter is typically used to form a toner image because such small spherical toner particles can be densely adhered to a latent electrostatic image. However, such a small spherical toner has a drawback in that a cleaning blade cannot properly scrape such small spherical toner particles since the toner particles often pass through the nip between the image bearing member and the cleaning blade, resulting in occurrence of bad cleaning of the surface of the image bearing member (namely, the charging roller is contaminated with toner particles). Therefore, it is necessary to clean the surface of the charging roller to prevent occurrence of various problems. 
   Specific examples of such cleaning members for use in such a charging roller include sponge materials such as a polyurethane foam and a polyethylene foam disclosed in unexamined published Japanese patent application No. 5-297690, and a brush roller disclosed in unexamined published Japanese patent application No. 2002-221883. Matters such as toner on the surface of the charging roller are removed when such a cleaning member is brought into contact with and abrade the surface of the charging roller. The removed matters are collected in pores inside such a sponge material or between fibers of brushes located on a brush roller. However, the amount of the matters which can be stored in such members is limited. Therefore, maintaining a good cleaning performance for a long period of time is a remaining issue. For example, in the case of a process cartridge including a charging roller, the charging roller needs to have a life length as long as those of other members constituting the process cartridges, each of which has a relatively long life. Therefore, a cleaning device having such a brush roller is not suitable for such process cartridges. 
   In addition, it is necessary for the cleaning device to remove foreign materials such as paper dust, which adhere to the charging roller. 
   Because of these reasons, the need exists for a long-life charging device having a cleaner which can efficiently clean materials electrostatically adhered to the surface of a charging roller. 
   SUMMARY OF THE INVENTION 
   Accordingly, an object of the present invention is to provide a long-life charging device with a cleaner which can efficiently clean materials electrostatically adhered to the surface of a charging roller. 
   Another object of the present invention is to provide a process cartridge and an image forming apparatus, which can produce high quality and high definition images over a long period of time. 
   Briefly these objects and other objects of the present invention as herein after will become more readily apparent can be attained by a charging device including a charging roller having a metal cylinder and an elastic layer located on the metal cylinder, and a cleaner configured to clean the surface of the charging roller. The cleaner includes a driving shaft and a cleaning roller which is rotatably mounted on the driving shaft. In addition, the cleaning roller is made of a non-cellular foam resin having a density of from 5 to 15 kg/m 3  and a tensile strength of from 1.2 to 2.2 kg/cm 2 . 
   It is preferred that the foam resin have an expansion rate of from 20 to 40%. 
   It is also preferred that the cleaning roller be made of a melamine foam resin. 
   It is also preferred that the cleaning roller be rotatably contacted with the charging roller such that the cleaning roller rotates together with the charging roller. 
   The cleaning roller preferably has an oscillating unit configured to oscillate the cleaning roller in the longitudinal direction thereof. 
   The cleaning roller can have a one-way clutch on the shaft thereof to slightly change the contact face of the cleaning roller with the charging roller. 
   As another aspect of the present invention, a process cartridge is provided which can be detachably attached to an image forming apparatus and which includes: 
   at least an image bearing member configured to bear a latent electrostatic image; and 
   the charging device mentioned above configured to charge the image bearing member. 
   As yet another aspect of the present invention, an image forming apparatus is provided which includes: 
   an image bearing member; 
   the charging device mentioned above configured to charge the image bearing member; 
   a light irradiator configured to irradiate the charged image bearing member with imagewise light to form a latent electrostatic image on the image bearing member; 
   a developing device configured to develop the electrostatic latent image with a developer including a toner to form a toner image on the image bearing member; 
   a transferring device configured to transfer the toner image onto a receiving material; and 
   a fixing device configured to fix the toner image on the receiving material. 
   The toner preferably has a volume average particle diameter (Dv) of from 3 to 8 μm, and a ratio (Dv/Dn) of the volume average particle diameter (Dv) to a number average particle diameter (Dn) of from 1.00 to 1.40. 
   In addition, each of the form factors SF- 1  and SF- 2  of the toner is preferably greater than 100 and not greater than 180. 
   The toner is preferably prepared by a method including: 
   dispersing or dissolving toner constituents including at least a polyester prepolymer having a functional group having a nitrogen atom, another polyester resin, a colorant, and a release agent in an organic solvent to prepare a toner constituent liquid; and 
   dispersing the toner constituent liquid in an aqueous medium including a compound capable of reacting the functional group of the polyester prepolymer to crosslink and/or elongate the polyester prepolymer and to form toner particles in the aqueous medium. 
   It is also preferred that the toner have a spherical form and satisfy the following relationships:
 
0.5≦ r 2/ r 1≦1.0; and
 
0.7≦ r 3/ r 2≦1.0,
 
wherein r 1  represents a major-axis particle diameter of the toner, r 2  represents a minor-axis particle diameter of the toner and r 3  represents a thickness of the toner, wherein r 3 ≦r 2 ≦r 1 . In this case, 100 particles of the toner are observed to determine the ratios r 2 /r 1  and r 3 /r 2 .
 
   These and other objects, features and advantages of the present invention will become apparent upon consideration of the following description of the preferred embodiments of the present invention taken in conjunction with the accompanying drawings. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Various other objects, features and attendant advantages of the present invention will be more fully appreciated as the same becomes better understood from the detailed description when considered in connection with the accompanying drawings in which like reference characters designate like corresponding parts throughout and wherein: 
       FIG. 1  is a schematic view illustrating the cross section of an image forming apparatus having an embodiment of the charging device of the present invention; 
       FIG. 2  is an enlarged view of the main portion of the image forming apparatus illustrated in  FIG. 1 ; 
       FIG. 3  is a schematic view illustrating an embodiment of the cleaner of the charging device of the present invention; 
     FIG.  4 - a  is a graph illustrating the relationships between the density of the foam resin and the image quality level in view of background fouling and streak; 
     FIG.  4 - b  is a graph illustrating the relationships between the tensile strength of the foam resin and the image quality level in view of background fouling and streak; 
       FIGS. 5A and 5B  are projected images of toner particles for explaining the form factors SF- 1  and SF- 2  thereof; 
       FIGS. 6A to 6C  are schematic views of a toner particle for explaining the major axis particle diameter, the minor axis particle diameter and the thickness of the toner particle; and 
       FIGS. 7 and 8  are schematic views of the cleaner and charging device according to exemplary embodiments of the invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   The present invention will be explained with reference to drawings. 
     FIG. 1  is a schematic view illustrating the cross section of an image forming apparatus having an embodiment of the charging device of the present invention.  FIG. 2  is an enlarged view of the main portion of the image forming apparatus illustrated in  FIG. 1 . An image forming apparatus (i.e., an electrophotographic copier)  100  includes a scanner unit  20  which reads the image of an original, an image forming unit  30  which reproduces the read image on a receiving material  5 , and a paper feeding unit  40  which timely feeds the receiving material  5  to the image forming unit  30 . The image forming unit  30  includes a photoreceptor  1  serving as an image bearing member, and a charging device  2 , a light irradiator  3 , a developing device  4 , a transferring device  6 , a fixing device  7  and a cleaning device  8 , which are arranged in the vicinity of the photoreceptor  1 . Numeral  9  denotes a discharger configured to irradiate the photoreceptor  1  with light to discharge charges remaining on the photoreceptor  1 . 
   The photoreceptor  1  includes a photoconductive material such as amorphous metals, e.g., amorphous silicon and amorphous selenium; and organic compounds such as bisazo pigments and phthalocyanine pigments. In view of environmental protection and post-treatment of the photoreceptor, the organic compounds are preferably used. 
   The charging device  2  has a charging roller  2   a  having a-metal cylinder and an elastic layer formed on the peripheral surface of the metal cylinder, a cleaner  2   b  and a power source (not shown) connected with the charging roller  2   a . The power source applies a high voltage to the charging roller  2   a  to form a predetermined high electric field at the charging portion in which the charging roller  2   a  faces the photoreceptor  1 . As a result, corona discharging occurs at the charging portion, and thereby the surface of the photoreceptor  1  is uniformly charged. 
   The cleaner  2   b  has a cleaning roller  2   c  configured to clean the surface of the charging roller  2   a . The cleaner  2   b  will be explained below in detail. 
   The light irradiator  3  converts the data, which are read by a scanner in the scanner unit  20  or which are sent from an external device such as personal computers, to image data. The light irradiator  3  irradiates the surface of the photoreceptor  1  with imagewise laser light  3   a  via an optical system (not shown) including a polygon mirror, mirrors, lens, etc. 
   The developing device  4  has a developer bearing member  4   a  which bears a developer including a toner to supply the developer to the photoreceptor  1 , a toner supplying room, a developer regulator configured to control the thickness of the developer layer formed on the developer bearing member  4   a  and other members. The developer bearing member  4   a  is arranged in a close vicinity to the photoreceptor  1  while a small gap is formed therebetween. 
   The developer bearing member  4   a  includes a cylindrical developer bearing member which is rotatably supported and a magnetic roller which is fixed inside the cylindrical developer bearing member so as to be coaxial to the cylindrical developer bearing member. The developer bearing member  4   a  transports the developer while bearing the developer on the peripheral surface using a magnetic force of the magnetic roller. The developer bearing member  4   a  is electroconductive and is made of a nonmagnetic material. In addition, a power source is connected with the developer bearing member  4   a  to apply a developing bias thereto. Namely, a voltage is applied to the developer bearing member  4   a  to form an electric field between the photoreceptor  1  and the developer bearing member  4   a.    
   The transfer device  6  includes a transfer belt  6   a , a transfer bias roller  6   b , and a tension roller  6   c . The transfer bias roller  6   b  has a metal cylinder and an elastic layer formed on the metal cylinder. When a toner image is transferred from the photoreceptor  1  to the receiving material  5 , a pressure is applied to the transfer bias roller  6   b  to press the receiving material  5  to the photoreceptor  1 . 
   The transfer belt  6   a  is a seamless belt made of a material having a high heat resistance, such as polyimide films. A fluorine-containing resin layer can be formed on the outermost surface of the transfer belt  6   a . In addition, a silicone rubber layer can also be formed between the base material of the transfer belt and the fluorine-containing resin layer. The tension roller  6   c  is provided to rotate the transfer belt  6   a  while tightly stretching the transfer belt  6   a.    
   The fixing device  7  includes a fixing roller having a heater such as halogen lamps therein and a pressure roller which is pressure-contacted with the fixing roller. The fixing roller has a metal cylinder, an elastic layer (e.g., silicone rubber layers) having a thickness of from 100 to 500 μm (preferably about 400 μm), and an outermost resin layer including a releasing resin such as fluorine-containing resins. The outermost resin layer is typically formed using a resin tube such as tetrafluoroethylene/perfluoroalkylvinyl ether copolymer (PFA) tubes. The thickness of the outermost resin layer is preferably from 10 to 50 μm. A temperature detector is provided on the peripheral surface of the fixing roller to measure the temperature of the surface of the fixing roller and to control the temperature so as to be in a range of from about 160° C. to about 200° C. 
   The pressure roller includes a metal cylinder and an offset preventing layer formed on the metal cylinder. The offset preventing layer is typically made of a material such as tetrafluoroethylene/perfluoroalkylvinyl ether copolymers (PFA) and polytetrafluoroethylene (PTFE). Similar to the fixing roller, an elastic layer can be formed between the metal cylinder and the offset preventing layer. 
   The cleaning device  8  for cleaning the photoreceptor  1  includes a first cleaning blade  8   a  and a second cleaning blade  8   b  which is located on the downstream side from the first cleaning blade  8   a  relative to the rotating direction of the photoreceptor  1 . In addition, the cleaning device  8  also includes a collection member  8   d  configured to collect the toner particles obtained by cleaning, a collection coil  8   c  configured to transport the collected toner particles, and a container (not shown) configured to contain the collected toner particles. 
   The first cleaning blade  8   a  is made of a material such as metals, resins and rubbers. Among these materials, rubbers such as fluorine-containing rubbers, silicone rubbers, butyl rubbers, butadiene rubbers, isoprene rubbers and urethane rubbers are preferably used. In particular, urethane rubbers are more preferably used. The first cleaning blade  8   a  mainly removes toner particles remaining on the surface of the photoreceptor  1  after the transferring process. 
   The second cleaning blade  8   b  mainly removes materials such as additives included in the toner, which adhere to the surface of the photoreceptor  1  like a film. The second cleaning blade  8   b  can be made of the same material as that of the first cleaning blade  8   a  but typically includes an abrasive to effectively remove the film materials formed on the photoreceptor  1 . 
   Then the cleaner  2   b  of the charging device  2  of the present invention which cleans the surface of the charging roller will be explained. 
   The cleaner  2   b  of the charging device  2  of the present invention includes the cleaning roller  2   c  which is made of a foam resin as a cleaning member. The foam resin, for example, can be wound on a metallic cylinder. The foam resin used is preferably a non-cellular foam resin having a density of from 5 to 15 kg/m 3  and a tensile strength of from 1.2 to 2.2 kg/cm 2 . 
     FIGS. 4-1  and  4 - 2  are graphs illustrating the image quality level while the density of the foam is changed and while the tensile strength of the foam is changed, respectively. 
   When the cleaning performance of the cleaning roller  2   c  is bad and dusts on the surface of the charging roller  2   a  are not removed, the photoconductor  1  is not charged well and therefore background fouling occurs. Sequential line graphs connecting squares plotted in  FIGS. 3-1  and  3 - 2  represent the relationship between the density of the foam and image quality level in terms of background fouling and the relationship between the tensile strength of the foam and image quality level in terms of background fouling, respectively. The less the background fouling, the higher the image quality level. 
   When abrasion between the cleaning roller  2   c  and the charging roller  2   a  causes scars on the surface of the charging roller  2   a , images obtained have streaks. Sequential line graphs connecting circles plotted in  FIGS. 3-1  and  3 - 2  represent the relationship between the density of the foam and image quality level in terms of streaks and the relationship between the tensile strength of the foam and image quality level in terms of streaks, respectively. The less the streak, the higher the image quality level. The highest image quality level is 5.0 and the practically acceptable image quality is not less than 3.0. 
   As seen from  FIG. 4-1 , when the foam has a density not less than 5 kg/m 3 , cleaning performance of the cleaning roller  2   c  is sufficient. In contrast, when the foam has a density less than 5 kg/m 3 , cleaning performance of the cleaning roller  2   c  becomes so poor that bad charging of the photoconductor  1  occurs at an early stage and as a result causes problems such as background fouling on the images obtained. To the contrary, when the density of the foam is greater than 15 kg/m 3 , cleaning performance is good but the effect of shaving the surface of the charging roller  2   a  is significant. Therefore, scars are made on the surface of the charging roller  2   a  at an early stage and leads to problems such as streaks on the images obtained. 
   In addition, as seen from  FIG. 4-2 , when the foam has a tensile strength not less than 1.2 kg/cm 2 , cleaning performance of the cleaning roller  2   c  is sufficient. When the foam has a tensile strength less than 1.2 kg/cm 2 , the strength is not enough and therefore the foam resin crumbles at an early stage and as a result sufficient cleaning is impossible. In contrast, when the foam has a tensile strength greater than 2.2 kg/cm 2 , the surface of the charging roller  2   a  is scarred at an early stage and images obtained have streaks regardless of cleaning ability, 
   Therefore, it is preferred that the foam resin constituting the cleaning roller  2   c  have a density of from 5 to 15 kg/M 3  and a tensile strength of from 1.2 to 2.2 kg/cm 2 . The foam resin having a continuous foam structure and a density within the range mentioned above, has a mesh form with fine pores. The cleaning roller  2   c  can shave off extraneous matters such as toners on the surface of the charging roller  2   a  with this bone structure of the foam. 
   In addition, the foam resin having a tensile strength within the range mentioned above, tends to crumble and therefore a portion of the foam resin where the foam resin contacts with the charging roller  2   a  may fall off by the frictional force therebetween. The extraneous matters such as toners contained in pores in the foam fall off together. That is, different from conventional foam resins, the foam resin does not store extraneous matters in the pores thereof and can clean the charging roller  2   a  with a fresh face of the foam. Consequently, the cleaning roller  2   c  maintains a good cleaning performance for a long period of time without scratching the surface of the charging roller  2   a.    
   Among the foam resins having the properties mentioned above, a melamine foam resin is especially preferred. The foam resins made of melamine resin has hard mesh fibers and therefore can shave off or hook and remove extraneous matters on the surface of the charging roller  2   a . Since the melamine foam resin has not only this excellent cleaning ability but also the tendency of crumbling mentioned above, a fresh face of the cleaning roller  2   c  always contacts with the surface of the charging roller  2   a . Therefore, excellent cleaning ability is maintained. 
   The cleaning roller  2   c  is rotatably supported and rotates interlockingly with the charging roller  2   a  in the direction shown by an arrow illustrated in  FIG. 2 . This means that, since the cleaning roller  2   c  is driven by the charging roller  2   a  and therefore the cleaning roller  2   c  does not require a driving device, the structure can be simplified. In addition, since the cleaning roller  2   c  is made of the foam resin mentioned above, a pressure to make the cleaning roller  2   c  contact with the surface of the charging roller  2   a  is not especially necessary for an excellent cleaning performance. As a result, wearing of the surface of the charging roller  2   a  can be restrained 
   In addition, the cleaner  2   b  preferably has an oscillating mechanism configured to oscillate the cleaning roller  2   c  in the longitudinal direction thereof according as the charging roller  2   a  rotates. For example, a bearing is provided on the top of the shaft of the cleaning roller  2   c  so as to face the surface of an oscillating cam of a gear. When the charging roller  2   a  rotates, the gear with the oscillating cam is also rotated, and thereby the cleaning roller  2   c  is oscillated in the longitudinal direction thereof. 
   By oscillating the cleaning roller  2   c , the surface of the charging roller  2   a  can be uniformly cleaned. In particular, paper dust is typically generated from both edge portions of receiving papers, and therefore paper dust is mainly adhered to edge portions of the photoreceptor  1 . The paper dust is then transferred to the edge portions of the charging roller  2   a . By oscillating the cleaning roller  2   c , such paper dust can be easily removed from the charging roller  2   a.    
   Alternatively, a one-way clutch can be provided on the shaft of the cleaning roller  2   c . During the image forming operations are performed, the one-way clutch is locked, i.e., the cleaning roller  2   c  does not rotate. Therefore, the charging roller  2   a  is cleaned while rubbed by the cleaning roller  2   c , which is not in rotation. When the image forming operations complete, the photoreceptor  1  stops after reversely rotating slightly. At this point, the cleaning roller  2   c  also slightly rotates via the one-way clutch and then stops. By using such a mechanism, the charging roller  2   a  can avoid contacting with the foam resin portion of the cleaning roller  2   c  under an excessive pressure and therefore wearing of the charging roller  2   a  can be restrained. In addition, the contact face of the cleaning roller  2   c  against the charging roller  2   a  is shifted little by little, and thereby cleaning can be well performed at any time. 
   The cleaner mentioned above for cleaning a charging roller can be used for not only an image forming apparatus but also a process cartridge which is detachable to the image forming apparatus and which includes at least a photoreceptor and a charger, optionally together with one or more devices such as developing devices and photoreceptor-cleaning devices. Specifically, the cleaner mentioned above for cleaning a charging roller is also provided on the charger of the process cartridge. The cleaner can clean the surface of the charging roller and maintain its cleaning ability until the life of the process cartridge comes to an end. Therefore charging is well performed over a long period of time. 
   The image forming apparatus of the present invention having the charging device with the cleaner is not limited to the embodiment mentioned above. For example, an image forming apparatus including an intermediate transfer medium which bears a toner image transferred from a photoreceptor to retransfer the toner image to a receiving material; an image forming apparatus including a plurality of photoreceptors to produce multi-color images; and the like apparatus can also be included in the scope of the present invention. 
   The toner for use in the image forming apparatus of the present invention preferably has a volume average particle diameter (Dv) of from 3 to 8 μm, and a ratio (Dv/Dn) of the volume average particle diameter (Dv) to the number average particle diameter (Dn) is preferably from 1.00 to 1.40. Namely, a toner having a relatively small particle diameter and a narrow particle diameter distribution is preferably used. By using a toner having a small particle diameter, the toner can be densely adhered to a latent electrostatic image without protruding from the latent image, and thereby high density and high quality image can be produced. By using a toner having a narrow particle diameter distribution, the charge quantity distribution of the toner particles can be uniformed, and thereby high quality images without background development can be produced. In addition, the transferability of the toner can also be improved, and thereby the quantity of the toner particles remaining on the photoreceptor can be reduced, resulting in extension of the life of the cleaner for cleaning the charging roller. 
   The toner for use in the image forming apparatus preferably has a spherical form such that the form factors SF- 1  and SF- 2  of the toner fall in the specific ranges mentioned below.  FIGS. 5  are schematic views for illustrating the form factors SF- 1  and SF- 2 . 
   As illustrated in  FIG. 5A , the form factor SF- 1  represents the degree of the roundness of a toner particle and is defined by the following equation (1):
 
 SF -1={( MXLNG ) 2 /(AREA)}×(100π/4)  (1)
 
wherein MXLNG represents a diameter of the circle circumscribing the image of a toner particle, which image is obtained by observing the toner particle with a microscope; and AREA represents the area of the image.
 
   When the SF- 1  is 100, the toner particle has a true spherical form. It can be said that as the SF- 1  increases, the toner form differs much from the true spherical form. 
   As illustrated in  FIG. 5B , the form factor SF- 2  represents the degree of the concavity and convexity of a toner particle, and is defined by the following equation (2):
 
 SF -2={(PERI) 2 /(AREA)}×(100/4π)  (2)
 
wherein PERI represents the peripheral length of the image of a toner particle observed by a microscope; and AREA represents the area of the image.
 
   When the SF- 2  is 100, the surface of the toner particle does not have concavity and convexity. It can be said that as the SF- 2  increases, the toner surface is much roughened. 
   The form factors SF- 1  and SF- 2  are determined by the following method:
     (1) a photograph of particles of a toner is taken using a scanning electron microscope (S-800, manufactured by Hitachi Ltd.); and   (2) particle images of 100 toner particles are analyzed using an image analyzer (LUSEX 3 manufactured by Nireco Corp.).   

   The toner for use in the image forming apparatus preferably has a form factor SF- 1  greater than 100 and not greater than 180 and a form factor SF- 2  greater than 100 and not greater than 180. When the toner has a particle form near the true spherical form, the contact area of a particle of the toner with another particle of the toner decreases, resulting in decrease of the adhesion between the toner particles, and thereby the toner has good fluidity. In addition, the contact area of a particle of the toner with the photoreceptor also decreases, resulting in decreases of the adhesion of the toner particle to the photoreceptor, and thereby the transferability of the toner improves. On the otherhand, a spherical toner having form factors SF- 1  and SF- 2  of 100 tends to invade into the gap between the first cleaning blade  8   a  and the photoreceptor  1 , and thereby the toner preferably has form factors SF- 1  and SF- 2  greater than 100. When the form factors SF- 1  and SF- 2  are too large, a toner scattering problem in that toner particles are scattered around toner images tends to occur, resulting in deterioration of the image qualities. Therefore, it is preferred that the form factors SF- 1  and SF- 2  do not exceed 180. 
   The toner for use in the image forming apparatus of the present invention is preferably prepared by the following method:
     (1) toner constituents including at least a polyester prepolymer having a functional group having a nitrogen atom, another polyester resin, a colorant and a release agent are dissolved or dispersed in an organic solvent to prepare a toner constituent liquid; and   (2) the toner constituent liquid is dispersed in an aqueous medium including a compound which can be reacted with the polyester prepolymer to crosslink and/or elongate the polyester prepolymer and to prepare toner particles.   

   Then the toner constituents and toner manufacturing method will be described in detail. 
   Modified Polyester Resin 
   The toner of the present invention includes a modified polyester resin (i) as a binder resin. The modified polyester resin (i) is preferably prepared by crosslinking and/or elongating a polyester prepolymer having a functional group having a nitrogen atom with a compound such as amines. The modified polyester resin (i) means a polyester resin having a group other than the ester group; or a polyester resin in which a resin component other than the polyester resin is bonded with the polyester resin through a covalent bonding or an ionic bonding. Specifically the modified polyester resin means polyester resins which are prepared by incorporating a functional group such as an isocyanate group, which can be reacted with a carboxyl group or a hydroxyl group, in the end portion of a polyester resin and reacting the polyester resin with a compound having an active hydrogen atom. 
   Suitable modified polyester resins for use as the modified polyester resin (i) include reaction products of a polyester prepolymer (A) having an isocyanate group with an amine (B) can be used. As the polyester prepolymer (A) having an isocyanate group, for example, polyesters prepared by a method in which a polycondensation product of a polyol (PO) and a polycarboxylic acid (PC) which has a group having an active hydrogen is reacted with a polyisocyanate (PIC) can be used. 
   Suitable groups having an active hydrogen include a hydroxyl group (an alcoholic hydroxyl group and a phenolic hydroxyl group), an amino group, a carboxyl group, a mercapto group, etc. Among these groups, alcoholic hydroxyl groups are preffered. 
   Suitable preferred polyols (PO) include diols (DIO) and polyols (TO) having three or more hydroxyl groups. It is preferable to use diols (DIO) alone or mixtures in which a small amount of a polyol (TO) is added to a diol (DIO). 
   Specific examples of the diols (DIO) include alkylene glycol (e.g., ethylene glycol, 1,2-propylene glycol, 1,3-propylene glycol, 1,4-butanediol and 1,6-hexanediol); alkylene ether glycols (e.g., diethylene glycol, triethylene glycol, dipropylene glycol, polyethylene glycol, polypropylene glycol and polytetramethylene ether glycol); alicyclic diols (e.g. 1,4-cyclohexane dimethanol and hydrogenated bisphenol A); bisphenols (e.g., bisphenol A, bisphenol F and bisphenol S); adducts of the alicyclic diols mentioned above with an alkylene oxide (e.g., ethylene oxide, propylene oxide and butylene oxide); adducts of the bisphenols mentioned above with an alkylene oxide (e.g., ethylene oxide, propylene oxide and butylene oxide); etc. 
   Among these compounds, alkylene glycols having from 2 to 12 carbon atoms and adducts of bisphenols with an alkylene oxide are preferable. More preferably, adducts of bisphenols with an alkylene oxide, or mixtures of an adduct of bisphenols with an alkylene oxide and an alkylene glycol having from 2 to 12 carbon atoms are used. 
   Specific examples of the polyols (TO) include aliphatic alcohols having three or more hydroxyl groups (e.g., glycerin, trimethylol ethane, trimethylol propane, pentaerythritol and sorbitol); polyphenols having three or more hydroxyl groups (trisphenol PA, phenol novolak and cresol novolak); adducts of the polyphenols mentioned above with an alkylene oxide; etc. 
   Suitable polycarboxylic acids (PC) include dicarboxylic acids (DIC) and polycarboxylic acids (TC) having three or more carboxyl groups. It is preferable to use dicarboxylic acids (DIC) alone or mixtures in which a small amount of a polycarboxylic acid (TC) is added to a dicarboxylic acid (DIC). 
   Specific examples of the dicarboxylic acids (DIC) include alkylene dicarboxylic acids (e.g., succinic acid, adipic acid and sebacic acid); alkenylene dicarboxylic acids (e.g., maleic acid and fumaric acid); aromatic dicarboxylic acids (e.g., phthalic acid, isophthalic acid, terephthalic acid and naphthalene dicarboxylic acids; etc. Among these compounds, alkenylene dicarboxylic acids having from 4 to 20 carbon atoms and aromatic dicarboxylic acids having from 8 to 20 carbon atoms are preferably used. 
   Specific examples of the polycarboxylic acids (TC) having three or more hydroxyl groups include aromatic polycarboxylic acids having from 9 to 20 carbon atoms (e.g., trimellitic acid and pyromellitic acid). 
   As the polycarboxylic acid (PC), anhydrides or lower alkyl esters (e.g., methyl esters, ethyl esters or isopropyl esters) of the polycarboxylic acids mentioned above can be used for the reaction with a polyol (PO). 
   Suitable mixing ratio (i.e., an equivalence ratio [OH]/[COOH]) of a polyol (PO) to a polycarboxylic acid (PC) is from 2/1 to 1/1, preferably from 1.5/1 to 1/1 and more preferably from 1.3/1 to 1.02/1. 
   Specific examples of the polyisocyanates (PIC) include aliphatic polyisocyanates (e.g., tetramethylene diisocyanate, hexamethylene diisocyanate and 2,6-diisocyanate methylcaproate); alicyclic polyisocyanates (e.g., isophorone diisocyanate and cyclohexylmethane diisocyanate); aromatic didicosycantes (e.g., tolylene diisocyanate and diphenylmethane diisocyanate); aromatic aliphatic diisocyanates (e.g., α, α, α′, α′-tetramethyl xylylene diisocyanate); isocyanurates; blocked polyisocyanates in which the polyisocyanates mentioned above are blocked with phenol derivatives, oximes or caprolactams; etc. These compounds can be used alone or in combination. 
   Suitable mixing ratio (i.e., [NCO]/[OH]) of a polyisocyanate (PIC) to a polyester having a hydroxyl group is from 5/1 to 1/1, preferably from 4/1 to 1.2/1 and more preferably from 2.5/1 to 1.5/1. When the [NCO]/[OH] ratio is too large, the low temperature fixability of the toner deteriorates. In contrast, when the ratio is too small, the content of the urea group in the modified polyesters decreases and thereby the hot-offset resistance of the toner deteriorates. 
   The content of the constitutional component of a polyisocyanate (PIC) in the polyester prepolymer (A) having a polyisocyanate group at its end portion is from 0.5 to 40% by weight, preferably from 1 to 30% by weight and more preferably from 2 to 20% by weight. When the content is too low, the hot offset resistance of the toner deteriorates and in addition the heat resistance and low temperature fixability of the toner also deteriorate. In contrast, when the content is too high, the low temperature fixability of the toner deteriorates. 
   The number of the isocyanate groups included in a molecule of the polyester prepolymer (A) is at least 1, preferably from 1.5 to 3 on average, and more preferably from 1.8 to 2.5 on average. When the number of the isocyanate group is too small (less than 1 per 1 molecule), the molecular weight of the resultant urea-modified polyester decreases and thereby the hot offset resistance deteriorates. 
   Specific examples of the amines (B), which are to be reacted with a polyester prepolymer, include diamines (B1), polyamines (B2) having three or more amino groups, amino alcohols (B3), amino mercaptans (B4), amino acids (B5) and blocked amines (B6) in which the amines (B1–B5) mentioned above are blocked. 
   Specific examples of the diamines (B1) include aromatic diamines (e.g., phenylene diamine, diethyltoluene diamine and 4,4′-diaminodiphenyl methane); alicyclic diamines (e.g., 4,4′-diamino-3,3′-dimethyldicyclohexyl methane, diaminocyclohexane and isophoron diamine); aliphatic diamines (e.g., ethylene diamine, tetramethylene diamine and hexamethylene diamine); etc. 
   Specific examples of the polyamines (B2) having three or more amino groups include diethylene triamine, triethylene tetramine. Specific examples of the amino alcohols (B3) include ethanol amine and hydroxyethyl aniline. Specific examples of the amino mercaptan (B4) include aminoethyl mercaptan and aminopropyl mercaptan. Specific examples of the amino acids (B5) include amino propionic acid and amino caproic acid. Specific examples of the blocked amines (B6) include ketimine compounds which are prepared by reacting one of the amines B1–B5 mentioned above with a ketone such as acetone, methyl ethyl ketone and methyl isobutyl ketone; oxazoline compounds, etc. Among these compounds, diamines (B1) and mixtures in which a diamine (B1) is mixed with a small amount of a polyamine (B2) are preferable. 
   The mixing ratio (i.e., a ratio [NCO]/[NHx]) of the content of the prepolymer (A) having an isocyanate group to the amine (B) is from 1/2 to 2/1, preferably from 1.5/1 to 1/1.5 and more preferably from 1.2/1 to 1/1.2. When the mixing ratio is too low or too high, the molecular weight of the resultant urea-modified polyester decreases, resulting in deterioration of the hot offset resistance of the resultant toner. 
   The modified polyesters may include an urethane bonding as well as a urea bonding. The molar ratio (urea/urethane) of the urea bonding to the urethane bonding is from 100/0 to 10/90, preferably from 80/20 to 20/80 and more preferably from 60/40 to 30/70. When the content of the urea bonding is too low, the hot offset resistance of the resultant toner deteriorates. 
   The modified polyesters (i) can be prepared, for example, by a method such as one-shot methods or prepolymer methods. The weight average molecular weight of the modified polyesters (i) is not less than 10,000, preferably from 20,000 to 10,000,000 and more preferably from 30,000 to 1,000,000. When the weight average molecular weight is too low, the hot offset resistance of the resultant toner deteriorates. The number average molecular weight of the modified polyesters is not particularly limited (i.e., the weight average molecular weight should be primarily controlled so as to be in the range mentioned above) when a polyester resin (ii) which is not modified is used in combination. Namely, controlling of the weight average molecular weight of the modified polyester resins has priority over controlling of the number average molecular weight thereof. However, when a modified polyester is used alone, the number average molecular weight is from 2,000 to 15,000, preferably from 2,000 to 10,000 and more preferably from 2,000 to 8,000. When the number average molecular weight is too high, the low temperature fixability of the resultant toner deteriorates, and in addition the gloss of full color images decreases when the toner is used for color toners. 
   In the crosslinking reaction and/or elongation reaction of a polyester prepolymer (A) with an amine (B) to prepare a modified polyester (i), a reaction inhibitor can be used if desired to control the molecular weight of the resultant modified polyester. Specific examples of such a reaction inhibitor include monoamines (e.g., diethyle amine, dibutyl amine, butyl amine and lauryl amine), and blocked amines (i.e., ketimine compounds) prepared by blocking the monoamines mentioned above. 
   Unmodified Polyester 
   The toner for use in the image forming apparatus of the present invention includes not only the modified polyester resins (i) mentioned above, but also an unmodified polyester (ii) serving as a binder resin of the toner. By using a combination of a modified polyester (i) with an unmodified polyester (ii), the low temperature fixability of the toner can be improved and in addition the toner can produce color images having high gloss. 
   Suitable unmodified polyesters (ii) include polycondensation products of a polyol (PO) with a polycarboxylic acid (PC). Specific examples of the polyol (PO) and the polycarboxylic acid (PC) are mentioned above for use in the modified polyester (i) In addition, specific examples of the suitable polyol (PO) and polycarboxylic acid (PC) are also mentioned above. 
   In addition, as the unmodified polyester (ii), polyester resins modified by a bonding (such as urethane linkage) other than a urea linkage, can also be used as well as the unmodified polyester resins which are not modified at all. 
   When a mixture of a modified polyester (i) with an unmodified polyester (ii) is used as the binder resin, it is preferable that the modified polyester (i) at least partially mixes with the unmodified polyester (ii) to improve the low temperature fixability and hot offset resistance of the resultant toner. Namely, it is preferred that the modified polyester (i) have a structure similar to that of the unmodified polyester (ii). The mixing ratio (i/ii) of a modified polyester (i) to an unmodified polyester (ii) is from 5/95 to 80/20, preferably from 5/95 to 30/70, more preferably from 5/95 to 25/75, and even more preferably from 7/93 to 20/80. When the addition amount of the modified polyester (i) is too small, the hot offset resistance of the resultant toner deteriorates and in addition it is hard to impart a good combination of high temperature preservability and low temperature fixability to the resultant toner. 
   The peak molecular weight of the unmodified polyester (ii) for use in the toner of the present invention is from 1,000 to 10,000, preferably from 2,000 to 8,000, and more preferably from 2,000 to 5,000. When the peakmolecular weight is too low, the high temperature preservability of the toner deteriorates. In contrast, when the peak molecular weight is too high, the low temperature fixability of the toner deteriorates. 
   It is preferable for the unmodified polyester (ii) to have a hydroxyl value not less than 5 mgKOH/g, preferably from 10 to 120 mgKOH/g, and more preferably from 20 to 80 mgKOH/g. When the hydroxyl value is too low, it is hard to impart a good combination of high temperature preservability and low temperature fixability to the resultant toner. 
   The unmodified polyester (ii) preferably has an acid value of from 1 to 5 mgKOH/g, and more preferably from 2 to 4 mgKOH/g. In particular, when a wax having a high acid value is used for the toner as a release agent, the binder resin preferably has a low acid value to impart good charging ability and a high resistivity to the resultant toner. 
   In the toner of the present invention, the binder resin (i.e., the modified polyester and the unmodified polyester) preferably has a glass transition temperature (Tg) of from 35 to 70° C., and preferably from 55 to 65° C. When the glass transition temperature is too low, the high temperature preservability of the toner deteriorates. In contrast, when the glass transition temperature is too high, the low temperature fixability of the toner deteriorates. Since a modified polyester resin is used as the binder resin, the resultant toner has better high temperature preservability than conventional toners including a polyester resin as a binder resin even if the modified polyester resin has a relatively low glass transition temperature. 
   Colorant 
   The toner of the present invention includes a colorant. 
   Suitable colorants for use in the toner of the present invention include known dyes and pigments. Specific examples of the colorants include carbon black, Nigrosine dyes, black iron oxide, Naphthol Yellow S, Hansa Yellow (10G, 5G and G), Cadmium Yellow, yellow iron oxide, loess, chrome yellow, Titan Yellow, polyazo yellow, Oil Yellow, Hansa Yellow (GR, A, RN and R), Pigment Yellow L, Benzidine Yellow (G and GR), Permanent Yellow (NCG), Vulcan Fast Yellow (5G and R), Tartrazine Lake, Quinoline Yellow Lake, Anthrazane Yellow BGL, isoindolinone yellow, red iron oxide, red lead, orange lead, cadmium red, cadmium mercury red, antimony orange, Permanent Red 4R, Para Red, Fire Red, p-chloro-o-nitroaniline red, Lithol Fast Scarlet G, Brilliant Fast Scarlet, Brilliant Carmine BS, Permanent Red (F2R, F4R, FRL, FRLL and F4RH), Fast Scarlet VD, Vulcan Fast Rubine B, Brilliant Scarlet G, Lithol Rubine GX, Permanent Red F5R, Brilliant Carmine 6B, Pigment Scarlet 3B, Bordeaux 5B, Toluidine Maroon, Permanent Bordeaux F2K, Helio Bordeaux BL, Bordeaux 10B, BON Maroon Light, BON Maroon Medium, Eosin Lake, Rhodamine Lake B, Rhodamine Lake Y, Alizarine Lake, Thioindigo Red B, Thioindigo Maroon, Oil Red, Quinacridone Red, Pyrazolone Red, polyazo red, Chrome Vermilion, Benzidine Orange, perynone orange, Oil Orange, cobalt blue, cerulean blue, Alkali Blue Lake, Peacock Blue Lake, Victoria Blue Lake, metal-free Phthalocyanine Blue, Phthalocyanine Blue, Fast Sky Blue, Indanthrene Blue (RS and BC), Indigo, ultramarine, Prussian blue, Anthraquinone Blue, Fast Violet B, Methyl Violet Lake, cobalt violet, manganese violet, dioxane violet, Anthraquinone Violet, Chrome Green, zinc green, chromium oxide, viridian, emerald green, Pigment Green B, Naphthol Green B, Green Gold, Acid Green Lake, Malachite Green Lake, Phthalocyanine Green, Anthraquinone Green, titanium oxide, zinc oxide, lithopone and the like. These materials are used alone or in combination. 
   The content of the colorant in the toner is preferably from 1 to 15% by weight, and more preferably from 3 to 10% by weight, based on total weight of the toner. 
   Master batch pigments, which are prepared by combining a colorant with a resin, can be used as the colorant of the toner for use in the image forming apparatus of the present invention. Specific examples of the resin for use in the master batch pigments or for use in combination with master batch pigments include the modified and unmodified polyester resins mentioned above; styrene polymers and substituted styrene polymers such as polystyrene, poly-p-chlorostyrene and polyvinyltoluene; styrene copolymers such as styrene-p-chlorostyrene copolymers, styrene-propylene copolymers, styrene-vinyltoluene copolymers, styrene-vinylnaphthalene copolymers, styrene-methyl acrylate copolymers, styrene-ethyl acrylate copolymers, styrene-butyl acrylate copolymers, styrene-octyl acrylate copolymers, styrene-methyl methacrylate copolymers, styrene-ethyl methacrylate copolymers, styrene-butyl methacrylate copolymers, styrene-methyl α-chloromethacrylate copolymers, styrene-acrylonitrile copolymers, styrene-vinyl methyl ketone copolymers, styrene-butadiene copolymers, styrene-isoprene copolymers, styrene-acrylonitrile-indene copolymers, styrene-maleic acid copolymers and styrene-maleic acid ester copolymers; and other resins such as polymethyl methacrylate, polybutyl methacrylate, polyvinyl chloride, polyvinyl acetate, polyethylene, polypropylene, epoxy resins, epoxy polyol resins, polyurethane resins, polyamide resins, polyvinyl butyral resins, acrylic resins, rosin, modified rosins, terpene resins, aliphatic or alicyclic hydrocarbon resins, aromatic petroleum resins, chlorinated paraffin, paraffin waxes, etc. These resins are used alone or in combination. 
   Charge Controlling Agent 
   The toner for use in the image forming apparatus of the present invention includes a charge controlling agent. 
   Specific examples of the charge controlling agent include known charge controlling agents such as Nigrosine dyes, triphenylmethane dyes, metal complex dyes including chromium, chelate compounds of molybdic acid, Rhodamine dyes, alkoxyamines, quaternary ammonium salts (including fluorine-modified quaternary ammonium salts) alkylamides, phosphor and compounds including phosphor, tungsten and compounds including tungsten, fluorine-containing activators, metal salts of salicylic acid, salicylic acid derivatives, etc. 
   Specific examples of the marketed products of the charge controlling agents include BONTRON® 03 (Nigrosine dyes), BONTRON® P-51 (quaternary ammonium salt), BONTRON® S-34 (metal-containing azo dye), E-82 (metal complex of oxynaphthoic acid), E-84 (metal complex of salicylic acid), and E-89 (phenolic condensation product), which are manufactured by Orient Chemical Industries Co., Ltd.; TP-302 and TP-415 (molybdenum complex of quaternary ammonium salt), which are manufactured by Hodogaya Chemical Co., Ltd.; COPY CHARGE® PSY VP2038 (quaternary ammonium salt), COPY BLUE® (triphenyl methane derivative), COPY CHARGE® NEG VP2036 and NX VP434 (quaternary ammonium salt), which are manufactured by Hoechst AG; LRA-901, and LR-147 (boron complex), which are manufactured by Japan Carlit Co., Ltd.; copper phthalocyanine, perylene, quinacridone, azo pigments and polymers having a functional group such as a sulfonate group, a carboxyl group, a quaternary ammonium group, etc. 
   The content of the charge controlling agent is determined depending on the species of the binder resin used, whether or not an additive is added and toner manufacturing method (such as dispersion method) used, and is not particularly limited. However, the content of the charge controlling agent is typically from 0.1 to 10 parts by weight, and preferably from 0.2 to 5 parts by weight, per 100 parts by weight of the binder resin included in the toner. When the content is too high, the toner has too large charge quantity, and thereby the electrostatic force of a developing roller attracting the toner increases, resulting in deterioration of the fluidity of the toner and decrease of the image density of toner images. 
   Release Agent 
   The toner for use in the image forming apparatus of the present invention includes a release agent. Suitable release agents include waxes having a melting point of from 50 to 120° C. When such a wax is included in the toner, the wax is dispersed in the binder resin and serves as a release agent at a location between a fixing roller and the toner particles. Thereby hot offset resistance can be improved without applying an oil to the fixing roller used. 
   In the present invention, the melting point of the release agents is measured by a differential scanning calorimeter (DSC). The maximum absorption peak is defined as the melting point. 
   Specific examples of the release agent include natural waxes such as vegetable waxes, e.g., carnauba wax, cotton wax, Japan wax and rice wax; animal waxes, e.g., bees wax and lanolin; mineral waxes, e.g., ozokelite and ceresine; and petroleum waxes, e.g., paraffin waxes, microcrystalline waxes and petrolatum. In addition, synthesized waxes can also be used. Specific examples of the synthesized waxes include synthesized hydrocarbon waxes such as Fischer-Tropsch waxes and polyethylene waxes; and synthesized waxes such as ester waxes, ketone waxes and ether waxes. Further, fatty acid amides such as 1,2-hydroxylstearic acid amide, stearic acid amide and phthalic anhydride imide; and low molecular weight crystalline polymers such as acrylic homopolymer and copolymers having a long alkyl group in their side chain, e.g., poly-n-stearyl methacrylate, poly-n-laurylmethacrylate and n-stearyl acrylate-ethyl methacrylate copolymers, can also be used. 
   The charge controlling agent, and the release agent can be kneaded with a masterbatch and a binder resin. In addition, the charge controlling agent, and the release agent can be added to an organic solvent when the toner constituent liquid is prepared. 
   External Additive 
   The thus prepared tonerparticles (i.e., the mother toner) may be mixed with an external additive to assist in improving the fluidity, developing property and charging ability of the toner particles. Suitable external additives include particulate inorganic materials. It is preferable for the particulate inorganic materials to have a primary particle diameter of from 5 nm to 2 μm, and more preferably from 5 nm to 500 nm. In addition, it is preferable that the specific surface area of such particulate inorganic materials measured by a BET method is from 20 to 500 m 2 /g. The content of the external additive is preferably from 0.01 to 5% by weight, and more preferably from 0.01 to 2.0% byweight, based on total weight of the toner composition. 
   Specific examples of such inorganic particulate materials include silica, alumina, titanium oxide, barium titanate, magnesium titanate, calcium titanate, strontium titanate, zinc oxide, tin oxide, quartz sand, clay, mica, sand-lime, diatom earth, chromium oxide, cerium oxide, red iron oxide, antimony trioxide, magnesium oxide, zirconium oxide, barium sulfate, barium carbonate, calcium carbonate, silicon carbide, silicon nitride, etc. 
   Among these particulate inorganic materials, a combination of a hydrophobic silica and a hydrophobic titanium oxide is preferably used. In particular, when a hydrophobic silica and a hydrophobic titanium oxide each having an average particle diameter not greater than 50 nm are used as an external additive, the electrostatic force and van der Waals&#39; force between the external additive and the toner particles are improved, and thereby the resultant toner has a proper charge quantity. In addition, even when the toner is agitated in a developing device, the external additive is hardly released from the toner particles, and thereby image defects such as white spots and image omissions are hardly produced. Further, the quantity of particles of the toner remaining on image bearing members can be reduced. 
   When particulate titanium oxides are used as an external additive, the resultant toner can stably produce toner images having a proper image density even when environmental conditions are changed. However, the charge rising properties of the resultant toner composition tend to deteriorate particularly when the addition amount of the particulate titanium oxide is greater than that of the particulate silica. However, when the content of the hydrophobized silica and hydrophobized titanium oxide is from 0.3 to 1.5% by weight based on the weight of the toner particles, the charge rising properties of the toner do not deteriorate. Namely, good images can be produced by the toner even after long repeated use. 
   Then the method for manufacturing the toner for use in the present invention will be explained. However, the manufacturing method is not limited thereto.
     (1) At first, toner constituents including a colorant, an unmodified polyester resin, a polyester prepolymer having an isocyanate group, and a release agent are dissolved or dispersed in an organic solvent to prepare a toner constituent liquid.   

   Suitable organic solvents include organic solvents having a boiling point less than 100° C. so that the solvent can be easily removed from the resultant toner particle dispersion. 
   Specific examples of the organic solvents include toluene, xylene, benzene, carbon tetrachloride, methylene chloride, 1,2-dichloroethane, 1,1,2-trichloroethane, chloroform, monochlorobenzene, dichloroethylidene, methyl acetate, ethyl acetate, methyl ethyl ketone, methyl isobutyl ketone, etc. These can be used alone or in combination. In particular, aromatic solvents such as toluene and xylene, and halogenated hydrocarbons such as 1,2-dichloroethane, chloroform and carbon tetrachloride are preferably used. 
   The addition quantity of the organic solvent is from 0 to 300 parts by weight, preferably from 0 to 100 parts by weight and more preferably from 25 to 70 parts by weight, per 100 parts by weight of the polyester prepolymer used.
     (2) Then the toner constituent liquid is emulsified in an aqueous medium in the presence of a surfactant and a particulate resin.   

   Suitable aqueous media include water, and mixtures of water with alcohols (such as methanol, isopropanol and ethylene glycol), dimethylformamide, tetrahydrofuran, cellosolves (such as methyl cellosolve) and lower ketones (such as acetone and methyl ethyl ketone). 
   The mixing ratio (A/T) of the aqueous medium (A) to the toner constituent liquid (T) is from 50/100 to 2000/100 by weight, and preferably from 100/100 to 1000/100 by weight. When the content of the aqueous medium is too low, the toner constituent liquid cannot be well dispersed, and thereby toner particles having a desired particle diameter cannot be produced. In contrast, when the content of the aqueous medium is too high, the manufacturing cost of the toner increases. 
   When the toner constituent liquid is dispersed in an aqueous medium, a dispersant can be preferably used to prepare a stable dispersion. 
   Specific examples of the surfactants include anionic surfactants such as alkylbenzene sulfonic acid salts, α-olefin sulfonic acid salts, and phosphoric acid salts; cationic surfactants such as amine salts (e.g., alkyl amine salts, aminoalcohol fatty acid derivatives, polyamine fatty acid derivatives and imidazoline), and quaternary ammonium salts (e.g., alkyltrimethyl ammonium salts, dialkyldimethyl ammonium salts, alkyldimethyl benzyl ammonium salts, pyridinium salts, alkyl isoquinolinium salts and benzethonium chloride); nonionic surfactants such as fatty acid amide derivatives, polyhydric alcohol derivatives; and ampholytic surfactants such as alanine, dodecyldi(aminoethyl)glycin, di)octylaminoethyle)glycin, and N-alkyl-N,N-dimethylammonium betaine. 
   By using a surfactant having a fluoroalkyl group, a good dispersion can be prepared even when a small amount of the surfactant is used. Specific examples of the anionic surfactants having a fluoroalkyl group include fluoroalkyl carboxylic acids having from 2 to 10 carbon atoms and their metal salts, disodium perfluorooctanesulfonylglutamate, sodium 3-{omega-fluoroalkyl(C6–C11)oxy}-1-alkyl(C3–C4) sulfonate, sodium 3-{omega-fluoroalkanoyl(C6–C8)-N-ethylamino}-1-propa nesulfonate, fluoroalkyl(C11–C20) carboxylic acids and their metal salts, perfluoroalkylcarboxylic acids and their metal salts, perfluoroalkyl(C4–C12)sulfonate and their metal salts, perfluorooctanesulfonic acid diethanol amides, N-propyl-N-(2-hydroxyethyl)perfluorooctanesulfone amide, perfluoroalkyl(C6–C10)sulfoneamidepropyltrimethylamm onium salts, salts of perfluoroalkyl(C6–C10)-N-ethylsulfonyl glycin, monoperfluoroalkyl(C6–C16)ethylphosphates, etc. 
   Specific examples of the marketed products of such surfactants having a fluoroalkyl group include SURFLON® S-111, S-112 and S-113, which are manufactured by Asahi Glass Co., Ltd.; FRORARD® FC-93, FC-95, FC-98 and FC-129, which are manufactured by Sumitomo 3M Ltd.; UNIDYNE® DS-101 and DS-102, which are manufactured by Daikin Industries, Ltd.; MEGAFACE® F-110, F-120, F-113, F-191, F-812 and F-833 which are manufactured by Dainippon Ink and Chemicals, Inc.; ECTOP® EF-102, 103, 104, 105, 112, 123A, 306A, 501, 201 and 204, which are manufactured by Tohchem Products Co., Ltd.; FUTARGENT® F-100 and F150 manufactured by Neos; etc. 
   Specific examples of the cationic surfactants having a fluoroalkyl group include primary, secondary and tertiary aliphatic amino acids, aliphatic quaternary ammonium salts (such as perfluoroalkyl(C6–C10)sulfoneamidepropyltrimethylamm onium salts), benzalkonium salts, benzetonium chloride, pyridiniumsalts, imidazoliniumsalts, etc., allofwhich have a fluoroalkyl group Specific examples of the marketed products thereof include SURFLON® S-121 (from Asahi Glass Co., Ltd.); FRORARDO FC-135 (from Sumitomo 3M Ltd.);UNIDYNE®DS-202 (fromDaikin Industries, Ltd.); MEGAFACE® F-150 and F-824 (from Dainippon Ink and Chemicals, Inc.); ECTOPO EF-132 (from Tohchem Products Co., Ltd.); FUTARGENT® F-300 (from Neos); etc. 
   Any particulate polymers, whether they are thermoplastic resins or thermo-curing resins, can be also used as long as the toner constituents can form an aqueous dispersant. Specific preferred examples of such particulate polymers include vinyl resins, polyurethane resins, epoxy resins, polyester resins, polyamide resins, polyimide resins, silicone resins, phenol resins, melamine resins, urea resins, aniline resins, ionomer resins, and polycarbonate resins. The resins mentioned above can be used in combination. 
   Among the resins mentioned above, considering easiness of obtaining an aqueous dispersant of a particulate polymer having a fine spherical form, vinyl resins, polyurethane resins, epoxy resins, polyester resins and their combinational use are preferred. Specific preferred examples of such vinyl resins include homopolymers or copolymers of a vinyl monomer. Specific examples of such homopolymers and copolymers include styrene-(meta)acrylic ester copolymers, styrene butadiene copolymers, (meta) acrylic acid-acrylic ester copolymers, styrene-acrylic nitride copolymers, styrene-anhydride maleic acid copolymers, styrene-(meta) acrylic copolymers. The average particle diameter of the particulate polymer is from 5 to 300 nm and preferably from 20 to 200 nm. 
   In addition, an inorganic dispersant can be added to the aqueous medium. Specific examples of the inorganic dispersants include tricalcium phosphate, calcium carbonate, titanium oxide, colloidal silica, hydroxyapatite, etc. 
   Further, it is possible to stably disperse toner constituents in an aqueous medium using a polymeric protection colloid in combination with the inorganic dispersants and/or particulate polymers mentioned above. Specific examples of such protection colloids include polymers and copolymers prepared using monomers such as acids (e.g., acrylic acid, methacrylic acid, α-cyanoacrylic acid, α-cyanomethacrylic acid, itaconic acid, crotonic acid, fumaric acid, maleic acid and maleic anhydride), acrylic monomers having a hydroxyl group (e.g., β-hydroxyethyl acrylate, β-hydroxyethyl methacrylate, β-hydroxypropyl acrylate, β-hydroxypropyl methacrylate, γ-hydroxypropyl acrylate, γ-hydroxypropyl methacrylate, 3-chloro-2-hydroxypropyl acrylate, 3-chloro-2-hydroxypropyl methacrylate, diethyleneglycolmonoacrylic acid esters, diethyleneglycolmonomethacrylic acid esters, glycerinmonoacrylic acid esters, N-methylolacrylamide and N-methylolmethacrylamide), vinyl alcohol and its ethers (e.g., vinyl methyl ether, vinyl ethyl ether and vinyl propyl ether), esters of vinyl alcohol with a compound having a carboxyl group (i.e., vinyl acetate, vinylpropionate andvinyl butyrate); acrylic amides (e.g, acrylamide, methacrylamide and diacetoneacrylamide) and their methylol compounds, acid chlorides (e.g., acrylic acid chloride and methacrylic acid chloride), and monomers having a nitrogen atom or an alicyclic ring having a nitrogen atom (e.g., vinyl pyridine, vinyl pyrrolidone, vinyl imidazole and ethylene imine). 
   In addition, polymers such as polyoxyethylene compounds (e.g., polyoxyethylene, polyoxypropylene, polyoxyethylenealkyl amines, polyoxypropylenealkyl amines, polyoxyethylenealkyl amides, polyoxypropylenealkyl amides, polyoxyethylene nonylphenyl ethers, polyoxyethylene laurylphenyl ethers, polyoxyethylene stearylphenyl esters, and polyoxyethylene nonylphenyl esters); and cellulose compounds such as methyl cellulose, hydroxyethyl cellulose and hydroxypropyl cellulose, can also be used as the polymeric protective colloid. 
   The dispersion method is not particularly limited, and low speed shearing methods, high speed shearing methods, friction methods, high pressure jet methods, ultrasonic methods, etc. can be used. Among these methods, high speed shearing methods are preferable because particles having a particle diameter of from 2 μm to 20 μm can be easily prepared. At this point, the particle diameter (2 to 20 μm) means a particle diameter of particles including a liquid). 
   When a high speed shearing type dispersion machine is used, the rotation speed is not particularly limited, but the rotation speed is typically from 1,000 to 30,000 rpm, and preferably from 5,000 to 20,000 rpm. The dispersion time is not also particularly limited, but is typically from 0.1 to 5 minutes. The temperature in the dispersion process is typically from 0 to 150° C. (under pressure), and preferably from 40 to 98° C.
     (3) At the same time when a toner constituent is dispersed in an aqueous medium, an amine (B) is added to the aqueous medium to be reacted with the polyester prepolymer (A) having an isocyanate group.   

   This reaction accompanies crosslinking and/or elongation of the molecular chains of the polyester prepolymer (A). The reaction time is determined depending on the reactivity of the amine (B) with the polyester prepolymer used, but is typically from 10 minutes to 40 hours, and preferably from 2 to 24 hours. The reaction temperature is from 0 to 150° C., and preferably from 40 to 98° C. In addition, known catalysts such as dibutyltin laurate and dioctyltin laurate, can be used for the reaction, if desired.
     (4) After the reaction, the organic solvent is removed from the resultant dispersion (emulsion, or reaction product), and then the solid components are washed and then dried. Thus, a mother toner is prepared.   

   In order to remove the organic solvent, all the system is gradually heated while agitated so as to form laminar flow. Then the system is strongly agitated in a certain temperature range, followed by solvent removal, to prepare a mother toner having a spindle form. 
   In this case, when compounds such as calcium phosphate which are soluble in an acid or alkali are used as a dispersion stabilizer, it is preferable to dissolve the compounds by adding an acid such as hydrochloric acid, followed by washing of the resultant particles with water to remove calcium phosphate therefrom. In addition, calcium phosphate can be removed using a zymolytic method.
     (5) Then a charge controlling agent is fixedly adhered to the mother toner. In addition, an external additive such as combinations of a particulate silica and a particulate titanium oxide is adhered to the mother toner to prepare the toner of the present invention.   

   Addition of the charge controlling agent and the external additive to the mother toner can be made using a known method using a mixer or the like. 
   By using this manufacturing method, the resultant toner can have a relatively small particle diameter and a narrow particle diameter distribution. By controlling the strong agitation during the solvent removing process, the shape of the toner can be controlled so as to be a desired form of from a rugby ball form to a true spherical form. In addition, the surface condition of the toner can also be controlled so as to be a desired surface of from a smooth surface and a roughened surface. 
   The toner for use in the image forming apparatus of the present invention has substantially a spherical form satisfying the following relationships:
 
0.5 ≦r 2 /r 1≦1.0; and
 
0.7 ≦r 3 /r 2≦1.0,
 
wherein r 1  represents a major-axis particle diameter of the toner, r 2  represents a minor-axis particle diameter of the toner and r 3  represents a thickness of the toner, wherein r 3 ≦r 2 ≦r 1 .
 
     FIGS. 6A to 6C  are schematic views illustrating a typical toner particle of the toner for use in the present invention. As illustrated in  FIGS. 6A to 6C , when the major-axis particle diameter of the toner is represented by r 1 , the minor-axis particle diameter of the toner is represented by r 2  and the thickness of the toner is represented by r 3 , the ratio (r 2 /r 1 ) is preferably from 0.5 to 1.0 and the ratio (r 3 /r 2 ) is preferably from 0.7 to 1.0. 
   When the ratio (r 2 /r 1 ) is too small (i.e., the particle form of the toner is apart from the true spherical form), the dot reproducibility and the transferability of the toner deteriorate, and thereby high quality image cannot be produced. In addition, when the ratio (r 3 /r 2 ) is to small, the transferability of the toner deteriorates because the toner has a flat form. In particular, it is preferable that the ratio (r 3 /r 2 ) is 1.0, because the toner can be rotated around the major axis thereof. In this case, the toner has good fluidity. 
   The particle diameters r 1 , r 2  and r 3  of a toner are determined by observing 100 toner particles with a scanning electron microscope while the viewing angle is changed. 
   The thus prepared toner can be used as a magnetic or non-magnetic one-component developer including no magnetic carrier. 
   When the toner is used for a two-component developer, the toner is mixed with a magnetic carrier. Suitable magnetic carriers include ferrite and magnetite including a divalent metal atom such as Fe, Mn, Zn and Cu. The volume average particle diameter of the carrier is preferably from 20 to 100 μm. When the particle diameter is too small, a problem in that the carrier tends to adhere to the photoreceptor during the developing process occurs. In contrast, when the particle diameter is too large, the carrier is not mixed well with the toner, and thereby the toner is insufficiently charged, resulting in formation of undesired images such as images with background development. 
   Among the carrier materials mentioned above, Cu-ferrite including Zn is preferable because of having a high saturation magnetization. However, the carrier is not limited thereto, and a proper carrier is selected depending on the developing device of the image forming apparatus of the present invention. 
   The surface of the carrier can be coated with a resin such as silicone resins, styrene-acrylic resins, fluorine-containing resins and olefin resins. Such a resin is coated on a carrier typically by the following method:
     (1) a coating resin is dissolved in a solvent to prepare a coating liquid; and   (2) the coating liquid is coated on carrier particles, for example, by a spraying method using a fluidized bed.   

   Alternatively, the resin can also be coated by the following method:
     (1) a resin is electrostatically adhered to the surface of carrier particles; and   (2) the resin is heated to be fixed on the surface of the carrier particles.   

   The thickness of the thus formed resin layer on the carrier particles is from 0.05 to 10 μm, and preferably from 0.3 to 4 μm. 
   Having now fully described the invention, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit and scope of the invention as set forth herein.