Patent Publication Number: US-6223015-B1

Title: Recording medium carrier system intermediate transfer unit

Description:
This is a divisional of application Ser. No. 09/016,785 filed Jan. 30, 1998, the disclosure of which is incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to an intermediate transfer unit used in an image formation apparatus using an electrophotographic method, such as a copying machine, a printer, and a facsimile. The present invention also relates to a recording medium carrier system applied to the image formation apparatus. 
     As for a copying machine, a printer, a facsimile and other image formation apparatuses respectively using electrophotography, primarily an image formation apparatus using a laser beam writing device, it is important to fix a toner image while carrying a recording medium at high speed in order to make good use of the apparatus. It is also important to provide a simple means for relieving a paper jam or other problems caused by such operation. 
     Generally, an image formation apparatus using electrophotographic technology is provided with a photoconductive drum having a photosensitive layer as the peripheral face, charge means for evenly charging the peripheral surface of the photoconductive drum, exposure means for selectively exposing the evenly charged peripheral surface to form an electrostatic latent image, developing means for applying toner as a developer to the electrostatic latent image formed by the exposure means to form a visible image (a toner image), and transfer means for transferring the toner image developed by the developing means onto a transfer medium such as paper. 
     For transfer means for transferring a toner image developed on a photoconductive drum on a transfer medium, such as paper, heretofore, there is known transfer means provided with an intermediate transfer belt to which a toner image formed on a photoconductive drum is transferred (primary transfer) and which further transfers (secondary transfer) the toner image onto a recording medium, and with a driving roller for circulating the intermediate transfer belt. 
     As for the above prior transfer means, there is a problem that since a distance between a primary transfer position and the driving roller is large, the amount of shrinkage of the intermediate transfer belt between the primary transfer position and the driving roller is increased and the travel speed of the intermediate transfer belt in the primary transfer position is unstable. As a result, it is difficult to acquire satisfactory primary transfer. 
     Further, according to the above prior transfer means, there is a problem that a transfer roller directly touches the joint of the intermediate transfer belt, staining a secondary transfer roller by toner accumulated in a step of the joint of the intermediate transfer belt, and causing toner to adhere to the rear of a recording medium in a subsequent secondary transfer. 
     Further, according to the above prior transfer means, there is a problem that when a thin line image is transferred onto a recording medium, the surface of which is smooth, a failure of the transfer of toner (a void) occurs. 
     Further, according to the above prior transfer means, there is a problem that even if transfer on a recording medium having a smooth surface is satisfactory, transfer on a recording medium having a rough surface is insufficient. Particularly, when multiple layers of toner are transferred as a multiple color image, a failure to transfer toner far from the surface of a recording medium occurs. 
     Further, according to the above prior transfer means, there is a problem that in primary or secondary transfer, the deterioration of transfer efficiency and the omission (void) of a part of a toner image in transfer occurs. Also, in secondary transfer, there is a problem that it is difficult to transfer on a recording medium the surface of which is extremely irregular, such as recycled paper and bond paper, without lacking a part of an image. There is also a problem that if toner having a high fluidity is used, toner is readily scattered in transfer. In particular, if a primary or secondary transfer means which functions as a transfer electrode for applying transfer voltage to a transfer position, is located in a position distant from its transfer position, a transfer electric field in the transfer position cannot be concentrated upon the transfer position, and a toner image is scattered due to electrostatic force. For example, if the intermediate transfer belt is wound on the photoconductive drum without means for substantially pressing the intermediate transfer belt on the photoconductive drum or a recording medium in a transfer position, the area in which the photoconductive drum and the intermediate transfer belt are in contact in a transfer position is large and the turbulence of a toner image due to mechanical force caused by slight difference in speed between both and others readily occurs. 
     Further, according to the above prior transfer means, a monolayer or multilayer belt in which a conductive, a semiconductive or an insulating resin layer is generally formed, at least as the surface layer, is used for the intermediate transfer belt. Thus, there is a problem that, since the surface is made of resin as described above, friction and scratches are readily generated. In particular, a large quantity of particulates of metallic oxide generally adhere to the surface of a toner particle as an additive and, since the above additive is much harder than resin constituting the surface of the intermediate transfer belt, it is readily embedded in the intermediate transfer belt. Further, a phenomenon (so-called filming) in which toner adheres to the intermediate transfer belt in the embedded point, mentioned above, occurs and deteriorates the image. For example, the transfer efficiency in primary or secondary transfer deteriorates and a void (i.e., the lack of a part of a toner image in transfer) occurs. Also, in secondary transfer, there is a problem that it is difficult to transfer on a recording medium having a surface that is extremely irregular, such as recycled paper and bond paper, without causing an imperfection in an image. 
     Further, according to the above prior transfer means, there is a problem that a void occurs in a part of a toner image transferred on the intermediate transfer belt in primary transfer, particularly the center. Also, in secondary transfer, there is a problem that it is difficult to transfer on a recording medium having an extremely irregular surface, such as recycled paper and bond paper, without causing an imperfect image, in addition to the above problem of a void. 
     Further, in an image formation apparatus for forming a full color image by overlapping plural colors, for example, the secondary transfer means is prevented from being stained by controlling the driving of the secondary transfer means for executing secondary transfer so that the secondary transfer means is not in contact with the intermediate transfer belt while images of each color are formed. Instead, the secondary transfer means is touched to the intermediate transfer belt after the final image is formed and, when secondary transfer is started after primary transfer is finished, an image on the intermediate transfer belt is not disturbed. However, there is a problem that when the intermediate transfer belt is vibrated, such as when the secondary transfer means is switched to a state in contact or not in contact with the intermediate transfer belt, the speed is varied and turbulence of an image occurs. 
     Further, according to the above prior transfer means, transferability in a primary transfer part is insufficient. Concretely, there are problems in the quantity of toner (the thickness of the layer), dispersion in resistance among each member, the variation of transfer efficiency due to the variation of resistance, a phenomenon of a void, and the stability of the density due to aging. 
     Further, according to the above prior transfer means, transferability in a secondary transfer part is insufficient. Concretely, there are problems in, the quantity of toner (the thickness of the layer), the type of a recording medium such as plain paper, a postal card, and OHP sheet, dispersion in resistance and the variation of resistance among each member, the variation of transfer efficiency due to the variation of resistance by environment, a phenomenon of a void, and the stability of the density due to aging. 
     Further, with respect to resistance, which is an important characteristic of a primary transfer member and a secondary transfer member, the above transfer means includes members having approximately the same variation of resistance due to environment are used for both the primary and secondary transfer members. Therefore, if members having a small variation of resistance due to environment are used for both primary and secondary transfer members, current may leak in a part not related to transfer and the failure of transfer may occur, particularly in a case where a recording medium, such as a postal card or an envelope smaller in size than the width of the secondary transfer member, is printed in an environment of low temperature and low humidity in which the resistance of the recording medium is higher than that of the secondary transfer member in a secondary transfer part. To avoid the above situation, it is possible to increase the resistance of the secondary transfer member and reduce leakage current. However, since a member having small variation of resistance due to environment generally has a large dispersion of the resistance, there is a problem that the nonuniformity of transfer partly occurs. 
     In the meantime, if members having large variation of resistance due to environment is used for both primary and secondary transfer members, no failure due to a leak of secondary transfer occurs because the resistance of the secondary transfer member changes approximately as the change of the resistance of a recording medium due to environment. However, voltage required in a primary transfer part in the environment of low temperature and low humidity causes the cost to increase. 
     Further, in a prior transfer means as disclosed in Japanese Patent Application No. Hei. 7-322667, an imperfect image is prevented from occurring at the simultaneous timing of primary transfer and secondary transfer by providing a conductive layer on the intermediate transfer belt and setting a relationship between resistance R T  of a part from a primary transfer bias applying power source to the conductive layer and apparent resistance R 1  in a primary transfer part so that R T &lt;R 1 . 
     According to above prior transfer means, it is difficult, depending upon environment and the type of paper, to prevent an imperfect image from occurring at the simultaneous timing of primary transfer and secondary transfer. Concretely, if current which flows in a secondary transfer is larger than current which flows in a primary transfer, the phenomenon is remarkable. 
     SUMMARY OF THE INVENTION 
     The present invention is made to solve the above problems, and an object thereof is to provide a recording medium carrier system which is capable of easily dealing with various troubles caused by high-speed carriage of recording paper. 
     Another object of the invention is to provide an intermediate transfer unit by which the travel speed of an intermediate transfer belt in a primary transfer position can be stabilized. 
     Still another object of the invention is to provide an intermediate transfer unit by which the rear of a recording medium is not stained using an intermediate transfer belt with a joint. 
     Still another object of the invention is to provide an intermediate transfer unit for enabling satisfactory transfer onto a recording medium having a smooth surface such as OHP or having a rough surface, such as bond paper. The object is also to provide an intermediate transfer unit for enabling satisfactory transfer onto a recording medium the surface of which is smooth, in an overall area in the direction of the shaft of a transfer roller. The object is also to provide a compact and low-cost intermediate transfer unit for enabling satisfactory transfer onto a recording medium having a rough surface and simultaneously for enabling the reduction of torque for driving a transfer roller. The object is further to provide an intermediate transfer unit for enabling satisfactory transfer onto a recording medium having either a rough or a smooth surface while simultaneously maintaining a high quality image over long term use. The object is furthermore to provide an intermediate transfer unit for enabling the formation of an image approximately uniform in color in any density area on a recording medium having either a rough or smooth surface. 
     Still another object of the invention is to provide an intermediate transfer unit for forming a satisfactory image, without the lack of a part of an image such-as a void in transfer. 
     Still another object of the invention is to provide an intermediate transfer unit enabling the stabilization of transferability (transfer efficiency) in a primary transfer part. 
     Still another object of the invention is to provide an intermediate transfer unit enabling the stabilization of transferability (transfer efficiency) in the secondary transfer part. 
     Still another object of the invention is to provide an intermediate transfer unit enabling the stabilization of transferability (transfer efficiency) in the secondary transfer part and the reduction of the capacity of the high-voltage power source. 
     Still another object of the invention is to provide an intermediate transfer unit which can prevent the deterioration of an image in simultaneous transfer of primary transfer and secondary transfer. 
     In order to achieve the above objects, according to a first aspect of the invention, in a recording medium carrier system, a paper feed mechanism for carrying a recording medium to a transfer part, a mechanism for transferring a toner image onto a recording medium, a mechanism for fixing the transferred toner image on the recording medium, and a mechanism for ejecting the recording medium from a fixing part are respectively constituted as an independent unit. 
     According to a second aspect of the invention, an intermediate transfer unit is provided with an intermediate transfer belt to which a toner image formed on a photoconductive drum is primarily transferred and which further secondarily transfers the toner image onto a recording medium, and with a driving roller for circulating the intermediate transfer belt and is characterized in that the above primary transfer position is arranged close to the driving roller. 
     According to the intermediate transfer unit of the second aspect, since the primary transfer position is arranged close to the driving roller, the shrinkage of the intermediate transfer belt between the primary transfer position and the driving roller is reduced, the travelling speed of the intermediate transfer belt in the primary transfer position is stable and, as a result, primary transfer in a satisfactory state is readily acquired. 
     According to a third aspect of the invention, an intermediate transfer unit is provided with an intermediate transfer belt with a joint to which a toner image formed on a photoconductive drum is primarily transferred by a primary transfer member and which further secondarily transfers the toner image onto a recording medium using a secondary transfer roller, and with a driving roller for circulating the intermediate transfer belt and is characterized in that an electric field in a direction in which the above toner is returned from the secondary transfer roller to the intermediate transfer belt is formed while the secondary transfer roller is pressed on the intermediate transfer belt when no image is formed, and the secondary transfer roller is detached when the joint of the intermediate transfer belt is opposite to the secondary transfer roller. 
     According to the intermediate transfer unit of the third aspect, it is possible to prevent toner from adhering to the secondary transfer roller due to direct contact thereof with the joint of the intermediate transfer medium. Therefore, the rear of a recording medium will not be stained enabling the intermediate transfer unit to satisfactory transfer the toner. 
     A fourth aspect of the invention includes an intermediate transfer unit provided with an intermediate transfer belt which receives toner image formed on a photoconductive drum and transferred by a primary transfer member and which secondarily transfers the toner image onto a recording medium using a secondary transfer roller, wherein a driving roller drives the intermediate transfer belt. The fourth aspect of the invention is characterized in that the intermediate transfer belt includes dispersed fluoric particulates, at least in the surface layer, and the secondary transfer roller is pressed on the intermediate transfer belt under the linear pressure of 27 gf/mm or less. 
     Also, in the above intermediate transfer unit, the hardness of the above secondary transfer roller is set to 70° or less, as measured by a Asker-C hardness meter. 
     Also, in the above intermediate transfer unit, plural types of additives different in a particle diameter are added in the above toner and the surface coverage of them is 2 or more. 
     Also, in the above intermediate transfer unit, the above toner image transferred on the above intermediate transfer belt is 1.5 mg/cm 2  or less per unit area in any density area. 
     According to the intermediate transfer unit of the fourth aspect of the invention, since the intermediate transfer belt has an excellent mold releasing property, toner is readily released in secondary transfer, and when a thin line image is transferred onto a recording medium having a smooth surface satisfactory transfer is enabled even if pressure applied to the toner is not fixed. Further, since the hardness of the secondary transfer roller is set to 70° or less, as measured by Asker-C hardness meter, the concentration of transfer pressure is avoided in a linear image on the intermediate transfer belt and the occurrence of a void can be reduced. 
     Also, according to the above intermediate transfer unit, since pressure applied to the toner is uniform when a thin line image is transferred onto a recording medium having a smooth surface, satisfactory transfer is enabled. 
     Also, according to the above intermediate transfer unit, since an additive with a relatively large particle diameter is added, the additive is not embedded in a mother particle for a long term but the fluidity is maintained and the quality of an image is stable, and since an additive with a relatively small particle diameter is added, the surface coverage is large compared with the added weight, and even if pressure applied to toner is not fixed when a thin line image is transferred onto a recording medium having a smooth surface satisfactory transfer is enabled. 
     Also, according to the above intermediate transfer unit, since the height of a toner layer is limited and pressure applied to toner is made uniform when a thin line image is transferred onto a smooth recording surface by forming a toner layer in any density area under the condition that the quantity of toner to be transferred secondarily is 1.5 mg/cm 2  or less, satisfactory transfer is enabled. 
     A fifth aspect of the invention includes an intermediate transfer unit provided with an intermediate transfer belt which receives a toner image formed on a photoconductive drum and transferred by a primary transfer member and which secondarily transfers the toner image onto a recording medium using a secondary transfer roller, wherein a driving roller drives the intermediate transfer belt. The fifth aspect of the present invention is characterized in that the toner is coated with an additive at the surface coverage of 2 or more and the above secondary transfer roller is pressed on the intermediate transfer belt under the linear pressure of 15 gf/mm or more. 
     Also, in the above intermediate transfer unit, the hardness of the above secondary transfer roller is set to 50° or more, as measured by Asker-C hardness meter. 
     Also, in the above intermediate transfer unit, plural types of additives different in a particle diameter arc added in the above toner. 
     Also, in the above intermediate transfer unit, the toner image transferred on the intermediate transfer belt is 1.5 mg/cm 2  or less per unit area in any density area. 
     According to the intermediate transfer unit of the fifth aspect of the invention, since toner is coated with a sufficient quantity of additive, the force of the toner which adheres to the intermediate transfer belt can be reduced, toner can be also transferred in a concave portion of a recording medium the surface of which is rough, and secondary transfer in a satisfactory state can be readily acquired. Further, since a recording medium having a rough surface is pressed on the intermediate transfer belt under sufficient linear pressure, the concave portion of the recording medium can be brought close to a toner image on the intermediate transfer belt, and secondary transfer in a satisfactory state can be readily acquired. 
     Also, according to the above intermediate transfer unit, since the increase of driving torque by the excessive broadening of a secondary transfer nip formed by the secondary transfer roller and the intermediate transfer belt can be prevented, a driving motor can be miniaturized and an intermediate transfer unit which does not require a large space and a high cost can be readily obtained. 
     Also, according to the above intermediate transfer unit, since an additive with a relatively large particle diameter is added, the additive is not embedded in a mother particle for a long term but the fluidity is maintained and the quality of an image is stable. Further, since an additive with a relatively small particle diameter is also added, the surface coverage is large compared with the added weight and satisfactory transfer onto a recording medium having a rough surface is enabled. 
     Also, according to the above intermediate transfer unit, the occurrence of irregular color due to the transfer failure of toner of a layer farthest from a recording medium is reduced by forming a toner layer in any density area under the condition that the quantity of toner to be transferred secondarily is 1.5 mg/cm 2  or less. 
     According to a sixth aspect of the invention, an intermediate transfer unit is provided with an intermediate transfer belt to which a toner image formed on a photoconductive drum is primarily transferred in a primary transfer position and which further secondarily transfers the toner image onto a recording medium in a secondary transfer position; primary transfer means arranged inside the intermediate transfer belt, the intermediate transfer belt being carried between the photoconductive drum and the primary transfer means in the primary transfer position; and backup means arranged inside the intermediate transfer belt and secondary transfer means arranged outside the intermediate transfer belt, the intermediate transfer belt being carried between the backup means and the secondary transfer means in the secondary transfer position, and is characterized in that the loose apparent density of the toner is set to 0.35 g/cc or more, the shape factor SF- 1  of the toner is set to 150 or less, and SF- 2  is set to 140 or less. 
     According to the intermediate transfer unit of the sixth aspect, a void is prevented from occurring in transfer by pressing the primary transfer means and the secondary transfer means onto the intermediate transfer belt in the respective transfer positions, and satisfactory transfer is enabled, even for a recording medium having an extremely irregular surface, such as recycled paper and bond paper. 
     According to a seventh aspect of the invention, an intermediate transfer unit is provided with an intermediate transfer belt to which a toner image formed on a photoconductive drum is primarily transferred in a primary transfer position and which further secondarily transfers the toner image onto a recording medium in a secondary transfer position, primary transfer means arranged inside the intermediate transfer belt, and secondary transfer means arranged outside the intermediate transfer belt, and is characterized in that the load of the secondary transfer position is larger than a load in the primary transfer position. 
     In the intermediate transfer unit of the seventh aspect, the ratio of the load in the secondary transfer position to the load in the primary transfer position is 1.5 or more. 
     According to the intermediate transfer unit of the seventh aspect, a void is prevented from occurring in transfer by pressing the primary transfer means on the intermediate transfer belt by a relatively small load. Satisfactory transfer is also enabled for a recording medium having an extremely irregular surface, such as recycled paper and bond paper, by pressing the secondary transfer means onto the intermediate transfer belt by a relatively large load. Further, the durability of the intermediate transfer belt can be enhanced. 
     According to an eighth aspect of the invention, an intermediate transfer unit is provided with an intermediate transfer belt for primarily transferring a toner image formed on a photoconductive drum and further, secondarily transferring the toner image onto a recording medium, primary transfer means arranged inside the intermediate transfer belt, and secondary transfer means arranged outside the intermediate transfer belt, and is characterized in that the hardness of the secondary transfer means is higher than that of the primary transfer means. 
     In the intermediate transfer unit of the eighth aspect, the hardness of the secondary transfer means is higher than that of the primary transfer means by 10 degrees or more, as measured by an Asker-C hardness meter. 
     According to the intermediate transfer unit of the eighth aspect of the invention, since the hardness of the primary transfer means is relatively low, a void is prevented from occurring in transfer. Since the hardness of the secondary transfer means is relatively high, satisfactory transfer is enabled for a recording medium having an extremely irregular surface and further, the turbulence of an image caused by switching the position of the secondary transfer means between positions in contact and not in contact with the intermediate transfer belt can be prevented. 
     According to a ninth aspect of the invention, an intermediate transfer unit is characterized in that a toner image formed on the photoconductive drum is primarily transferred onto an intermediate transfer belt by supplying bias from a high-voltage power source to a primary transfer member arranged at the rear of the intermediate transfer belt, the resistance of the primary transfer member is set to 10 6  to 10 8  Ω, the surface resistivity of the intermediate transfer belt is set to 10 8  to 10 12  Ω, the volume resistivity is set to 10 8  to 10 12  Ωcm, the high-voltage power source has constant- current control when impedance in the primary transfer part is large and has constant-voltage control when the impedance is small. 
     According to the intermediate transfer unit of the ninth aspect of the invention, the control of the high-voltage power source is optimized. Therefore, since control under fixed current is executed in the case of a printing pattern in which 2 to 4 toner layers are overlapped, that is, when impedance is large, a required transfer electric field is secured every toner layer. In the meantime, since control under fixed voltage is executed in the case of a pattern in which the ratio of printing is small, that is, when impedance is small, a required and minimum electric field for transferring toner is secured. Also, since the resistance of the primary transfer member and the intermediate transfer belt is optimized, transfer is enabled at required and minimum voltage and current, and an imperfect image caused, for example, by abnormal discharge, can be prevented. 
     Also, since the hardness of the primary transfer member and a load onto the photoconductive drum by the primary transfer member are optimized, the dislocation of an image in primary transfer is prevented and a void can be prevented from occurring. 
     Also, a void can be prevented by optimizing both the quantity of an additive having a small particle diameter and the quantity of an additive having a large particle diameter. The two types of additives different in a particle diameter added to toner secure the fluidity of the toner and inhibit the deterioration of density due to aging. 
     According to a tenth aspect of the invention, an intermediate transfer unit is characterized in that a toner image formed on a photoconductive drum is primarily transferred onto an intermediate transfer belt, the toner image is secondarily transferred onto a recording medium by supplying bias from a high-voltage power source to a secondary transfer member pressed onto the backup roller, the resistance of the secondary transfer member is set to 10 6  to 10 8  Ω, the surface resistivity of the intermediate transfer belt is set to 10 8  to 10 12  Ω, the volume resistivity is set to 10 8  to 10 12  Ωcm, the high-voltage power source has constant-current control when impedance in the secondary transfer part is large and has constant-voltage control when the impedance is small. 
     According to the intermediate transfer unit of the tenth aspect of the invention, the control of the high-voltage power source is optimized. Therefore, when impedance is large, as in transferring onto a recording medium in environment in which temperature and humidity are low onto an OHP sheet, a transfer electric field required for constant-current control is secured and high transfer efficiency is maintained. In the meantime, since constant-voltage control is executed when impedance is small, such as in transferring onto a recording medium in a high temperature and humidity environment and where a width of a recording medium is narrower than that of the secondary transfer member, a required and minimum electric field for transferring toner is secured. Also, since the resistance of the secondary transfer member and the intermediate transfer belt is optimized, transfer is enabled at required and minimum voltage and current, thus preventing an imperfect image due to, for example, abnormal discharge. 
     Also, since the hardness of the secondary transfer member and a load onto the backup roller by the secondary transfer member are optimized, the dislocation of an image in secondary transfer is prevented and satisfactory transfer is also enabled onto a recording medium having a rough surface, such as bond paper. 
     Also, a void can be prevented from occurring by optimizing both the quantity of an additive with a small particle diameter and the quantity of an additive having a large particle diameter. The two types of additives different in a particle diameter added to the toner secure the fluidity of the toner and inhibit the deterioration of density due to aging. 
     According to an eleventh aspect of the invention, an intermediate transfer unit for primarily transferring a toner image formed on a photoconductive drum onto an intermediate transfer belt by supplying bias from a high-voltage power source to a primary transfer member arranged at the rear of the intermediate transfer belt and secondarily transferring the toner image onto a recording medium by supplying bias from a high-voltage power source to a secondary transfer member pressed on a backup roller, is characterized in that the primary transfer member and the secondary transfer member are formed by an elastic body, and the variation of the resistance of the secondary transfer member due to environment is set so that it is larger than that of the primary transfer member. 
     According to the intermediate transfer unit of the eleventh aspect of the invention, the change of the resistance of the primary transfer member and the secondary transfer member due to environment is optimized. Since the primary transfer member is made of a member having small change of resistance due to the environment, the required capacity of a primary transfer power source can be reduced. In the meantime, since the secondary transfer member is made of a member having large change of resistance due to the environment, no failure of transfer occurs in either of a low temperature and low humidity environment or in a high temperature and high humidity environment because the resistance changes approximately to that of a recording medium, such as paper. 
     According to a twelfth aspect of the invention, an intermediate transfer unit primarily transfers a toner image formed onto a photoconductive drum onto an intermediate transfer belt by applying bias from a high-voltage power source to a primary transfer member arranged in a position different from a primary transfer part on the surface of the intermediate transfer belt, and secondarily transfers the toner image onto a recording medium by applying bias to a secondary transfer member, and is characterized in that a backup member in the primary transfer part is an elastic body, the resistance of the primary transfer member is set to 1 MΩ or less, and a high-voltage power source for applying bias to the primary transfer member has current absorbable constant-voltage control. 
     According to a thirteenth aspect of the invention, an intermediate transfer unit primarily transfers a toner image formed on a photoconductive drum onto an intermediate transfer belt by applying bias from a high-voltage power source to a primary transfer member arranged in a position different from a primary transfer part on the surface of the intermediate transfer belt, and secondarily transfers the toner image onto a recording medium by applying bias to a secondary transfer member, and is characterized in that a backup member in the primary transfer part is an elastic body, the resistance of the primary transfer member is set to 1 MΩ or less, and a resistor is connected to a high-voltage power source, which applies bias to the primary transfer member, in parallel. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a block diagram of an apparatus showing an embodiment of the present invention. 
     FIG. 2 is a timing chart showing the operation of the above apparatus. 
     FIG. 3 is a schematic drawing showing an example of an image formation apparatus using an embodiment of an intermediate transfer unit according to the present invention. 
     FIG. 4 is a side view omitting a part and mainly showing the intermediate transfer unit. 
     FIG. 5 shows the main part of a gear train. 
     FIGS.  6 ( a ) to  6 ( c ) show an example of the particle size distribution of toner in the present invention. 
     FIG. 7 is a side view omitting a part mainly showing an intermediate transfer unit of an embodiment of the present invention. 
     FIG. 8 explains the function of an embodiment of the present invention. 
     FIG. 9 explains the function of an embodiment of the present invention. 
     FIG. 10 explains the function of an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Preferred embodiments of the present invention will now be described below. 
     FIG. 1 shows the outline of a color image formation apparatus provided with a recording medium carrier system of an embodiment of the present invention. 
     First, the whole system of the apparatus will be described. Around a photoconductive drum  2  in FIG. 1, in the order from the upstream side in the rotational direction, there are provided a charging roller  3 , a laser beam scanning type latent image formation unit  4 , developing units of yellow, magenta, cyan and black  5 ,  6 ,  7  and  8 , and a cleaning unit  10  opposite to a first transfer part  9 . The above apparatus is constructed so that a toner image according to recording information is formed on the photoconductive drum  2  by repeating each imaging process of yellow, magenta, cyan and black every rotation of the photoconductive drum  2 . 
     In the meantime, an intermediate transfer belt  11  is constructed so that a color toner image formed on the peripheral surface of the photoconductive drum  2  is transferred onto the intermediate transfer belt by primary transfer roller  12  and is secondarily transferred onto a recording medium S by a backup roller  13 . Recording paper S piled on a paper supply cassette  20  reaches a secondary transfer part via a pickup roller  24  and pairs of paper carriage rollers  31  and  33 , and in the secondary transfer part, a color toner image is transferred onto the recording paper. Further, after the transferred color toner image is fixed by a fixing unit  50 , the recording paper is ejected onto a paper ejection tray  66  via pairs of paper ejecting rollers  62  and  64 . 
     Next, a recording paper carrier mechanism will be described in detail. The paper supply cassette  20  is constructed so that it can be installed in the lower part at the front of the frame  1  of the apparatus, that is, in the lower part in FIG. 1, and the fixing unit  50  can be turned forward so that recording paper S can be readily supplied and measures for paper jam can be taken. 
     A paper pushing-up plate  21  provided to the above paper supply cassette  20  is coupled to a driving motor via a stepping clutch not shown and stopped at 120° and 240° so that the paper pushing-up plate is driven by the single driving motor. The driving motor also drives a cam  45  for touching or detaching a secondary transfer roller  41  and all the pair of paper separating rollers  26  and the pairs of paper carriage rollers  31  and  33  between the pickup roller  24  and the pair of gate rollers  35 . The paper pushing-up plate is constituted so that it is lifted when the whole apparatus starts operation and lowered after printing operation is finished. Further, a pressing roller  22  made of resin for pressing an envelope and others is provided in the paper supply cassette  20  at the back of the pickup roller  24 , so that slanting of the paper supply, which may be caused because the edge of the uppermost envelope of piled ones is lifted and is slantwise touched to the pickup roller  24 , can be prevented. 
     In the meantime, the pickup roller  24  for feeding recording paper S pushed up by the paper pushing-up plate  21  is approximately 40 mm long, is made of rubber having a hardness of 25° to 40°, and is constituted so that the pickup roller comes in contact with the center (in width) of paper. The pickup roller  24  is driven via a first clutch (not shown) so that the pickup roller is interlocked with the pair of paper separating rollers  26 . 
     The pair of paper separating rollers  26  is arranged close to the pickup roller  24  on the downstream side (i.e., in a direction in which paper is fed) of the pickup roller and consists of an upper separating roller  27  rotated in the carriage direction of paper and a lower separating roller  28  normally rotated and reversely rotated via a torque limiter. Both are respectively formed as a roller approximately 40 mm long so that each roller contacts the center (in width) of recording paper S and plural sheets are prevented from being fed. 
     In the meantime, a paper carriage path between the pair of paper separating rollers  26  and the secondary transfer part functions as a paper reversing carriage path  30  for reversing recording paper S. In this portion, first and second pairs of carriage rollers  31  and  33  and a pair of gate rollers  35  are arranged at an interval at which a postal card can be fed longitudinally or, according to circumstances, are arranged at an interval at which an envelope can be fed sideways, and are constituted so that driving force is transmitted via a second clutch. 
     The first pair of carriage rollers  31  are arranged close on the downstream side of the pair of paper separating rollers  26  and have a length equal to the width of recording paper S to supplement the unstable feeding of the pair of paper separating rollers  26 , which hold only the center (in width) of paper. 
     The pair of gate rollers  35  are supported by a plain bearing, whereas the first and second pairs of carriage rollers  31  and  33  are supported by a ball bearing. The above rollers are constituted so that the free rotation torque of these pairs of carriage rollers is smaller than that of the pair of gate rollers  35  and even if recording paper S fed at high speed collides with the pair of gate rollers  35 , the pair of gate rollers  35  are not moved by the force of the collision. 
     Further, in the paper reversing carriage path  30 , tensile force is prevented from being applied to recording paper S in the carriage process by setting the peripheral speed of each pair of rollers  26 ,  31  and  33  between the pickup roller  24  and the pair of gate rollers  35  so that it is slower, in order. Furthermore, slippage of the recording paper S in the secondary transfer part is prevented by setting the peripheral speed of the pair of gate rollers  35  so that it is faster than that of the transfer belt  11 . 
     The peripheral speed of each pair of rollers  26 ,  31  and  33  is set only to an extent that the peripheral speed of the roller on the downstream side is equal to or slower than the peripheral speed of the roller on the upstream side when the tolerances of the diameter of the rollers on the upstream side and the downstream side are at a maximum. Also, the peripheral speed of the pair of gate rollers  35  is set such that the peripheral speed of the gate roller is equal to or faster than the speed of the transfer belt  11  when the tolerance of the diameter of the gate roller is at a minimum. 
     In the paper reversing carriage path  30 , first and second paper sensors  32  and  34  are arranged close on the downstream side of the first pair of carriage rollers  31  and close on the upstream side of the pair of gate rollers  35 . If recording paper does not reach the first paper sensor  32  after a predetermined time elapses, measured from a point in time at which the pickup roller  24  starts the feed of the recording paper, a signal is output to control means independent of an abnormality sensed by paper sensor  34 . Therefore, the quantity of information to be sent to the control means is reduced. 
     The pickup roller  24 , the pair of paper separating rollers  26 , the first and second pairs of carriage rollers  31  and  33 , and the pair of gate rollers  35  described above are assembled as one paper feed unit  37  as shown by a broken line in FIG.  1 . The paper feed unit is attached to the body  1  of the apparatus so that it can be detached from the body, and is constituted so that it can be also connected to a paper supply cassette with large capacity. 
     In the meantime, reference number  40  denotes a secondary transfer roller unit arranged on the downstream side of the pair of gate rollers  35  via a paper guide member  38 . The unit  40  is constituted by a swing lever  41  which can be swung around a supporting point  43  with the swing lever biased by the spring  46  so at the secondary transfer roller  42  supported by the swing lever is in contact with the transfer belt  11 , and the cam  45  for swinging the swing lever  41  so that the secondary transfer roller  42  is disengaged from the transfer belt  11  via a cam follower  44 . 
     The cam  45  for touching or detaching is coupled to the driving motor via the stepping clutch (not shown) so that the cam is stopped in plural positions in one rotation, at 120° and 240°, in this embodiment, and the lead of the cam is formed to have an extremely small sine curve so that the secondary transfer roller is detached from the transfer belt  11  in a range in which atmospheric discharge may occur by applying voltage to the transfer belt  11 , for example, approximately 1 mm. 
     Due to the above construction, shock caused when the secondary transfer roller  42  contacts the transfer belt  11  is reduced and the deterioration of the quality of an image due to the shock is prevented. The application of voltage to the secondary transfer roller  42  is controlled so that after the secondary transfer roller  42  comes in contact with the transfer belt  11 , current application is started and before the secondary transfer roller  42  is detached, current application is stopped to prevent atmospheric discharge from occurring. 
     Reference number  50  denotes a fixing unit for fixing a transferred toner image on recording paper S. The fixing unit  50  is attached so that it can be turned outside with a supporting part  51  provided at the inner lower end as a supporting point and is constructed so that paper jams caused in the paper ejecting path can be easily handled and each developing unit  5  to  8  can be easily replaced. 
     The fixing unit  50  includes a heat roller  52 , first and second pressurizing rollers  54  and  56  pressed on the heat roller  52 , and a heat insulating member  55  arranged among them. Toner can be more securely fixed at higher speed by providing large nip length and large contact pressure to the first pressurizing roller  54  to melt the toner, providing large curvature to the second pressurizing roller  56  to fix the toner, and providing for guiding the recording paper and for controlling heat radiation from the heat roller  52  to the heat insulating member  55 . 
     A group of pairs of paper ejecting rollers are positioned downstream of the fixing unit  50 . As shown, for example, in FIG. 1, two pairs of paper ejecting rollers  62  and  64  in this embodiment are attached to the front side of the apparatus  1  as one paper ejecting roller unit. 
     These pairs of paper ejecting rollers  62  and  64  are constricted so that recording paper can be ejected on the paper ejection tray  66  with the recording paper S under tension by setting the paper carriage speed of these pairs of paper ejecting rollers  62  and  64  so that it is faster than that of the fixing unit  50  and setting the paper carriage speed of the pair of paper ejecting rollers  64  on the downstream side in the paper carriage direction so that it is faster than that of the pair of paper ejecting rollers  62  on the upstream side. 
     The peripheral speed of each pair of paper ejecting rollers  62  and  64  has only to be set to an extent that the peripheral speed of a roller on the downstream side, having a maximum diameter, including tolerances, is greater than or equal to the peripheral speed of a roller on the upstream side, having a minimum diameter, including tolerances. Reference numbers  61  and  63  each denote a paper detecting sensor arranged on the paper ejecting path. 
     Next, the recording paper carriage operation of the above apparatus will be described referring to FIG.  2 . 
     When the operation of the whole apparatus is started at time “a” after a period for initialization for supplying paper, the paper pushing-up plate  21  pushes up loaded recording paper S and touches the center in width of the uppermost paper to the pickup roller  24 . 
     When a paper feed/separating roller clutch is connected at time “b” in relation to an imaging process the rotation of the pickup roller  24  is started and feeds recording paper S, to the pair of paper separating rollers  26  arranged close on the downstream side of the pickup roller. The paper feed/separating roller clutch prevents plural sheets from being fed by rotating the lower separating roller  28  reversely. A paper carriage roller clutch connected together with the paper feed/separating roller clutch transmits rotation to each first and second pair of carriage rollers  31  and  33  for a time corresponding to the length of a paper path between the paper supply tray  20  and the pair of gate rollers  35 , that is, time “c”. The first and second pair of carriage rollers  31  and  33  contact the full width of recording paper S from the pair of paper separating rollers  26  to carry the recording papers to the pair of gate rollers  35  in a stable state. 
     At time “d” after a fixed time elapses after primary transfer is started, a gate roller clutch transmits driving force to the pair of gate rollers  35  for a time corresponding to the length of a path between the pair of gate rollers  35  and the secondary transfer roller  42 , that is, time “e”, and at the same time, carries recording paper S to a transfer part in cooperation with the first and second pairs of carriage rollers  31  and  33  to which the driving force is transmitted via the paper carriage roller clutch, then executes required transfer processing on recording paper S. 
     Though different according to the length in the carriage direction of recording paper S, the paper feed/separating roller clutch for carrying a second recording paper S is connected at time “f” before or after the operation of the gate roller clutch, at the following time “g”, the paper carriage roller clutch transmits driving force to the first and second pairs of carriage rollers  31  and  33  for time corresponding to a length of a path between the first pair of carriage rollers  31  and the pair of gate rollers  35 , that is, time “h”, and carries second recording paper S to the pair of gate rollers  35 . 
     In the meantime, in such an apparatus in which recording paper is continuously carried, high durability and advanced paper carriage control means are provided. However, the wear and tear of parts and the occurrence of paper jams, for example, cannot be avoided. If such a situation occurs, a target unit selected, for example, from one of independently attached units such as the paper feed unit  37 , a transfer unit  40 , the fixing unit  50 , and a paper ejecting unit  60  may be detached from the body  1  of the apparatus and inspected by a user to determine whether that component needs to be replaced. 
     As described above, according to the present invention, since a paper feed mechanism, a transfer mechanism, a fixing mechanism, and a paper ejecting mechanism constituting a recording medium carrier system are constructed as independent units, a user can handle a situation such as a paper jam or the wear and tear of parts, by detaching or replacing individual units. Thus, the cost required for maintenance can be reduced and the operation rate of the apparatus can be greatly enhanced. 
     FIG. 3 is a schematic drawing showing an example of an image formation apparatus using an embodiment of an intermediate transfer unit according to the present invention. 
     First, the image formation apparatus will be described briefly below, followed by a detailed description of the intermediate transfer unit. 
     A full color image can be formed using developing machines for four colors of toner of yellow, cyan, magenta and black by the above image formation apparatus. 
     In FIG. 3, reference number  150  denotes a case of the body of the apparatus and in case  150 , are provided an exposure unit  160 , a paper supply unit  70 , a photoconductor unit  100 , a developing unit  200 , an intermediate transfer unit  300 , a fixing unit  400 , a control unit  80  for controlling the whole apparatus. 
     The photoconductor unit  100  is provided with a photoconductive drum  110 , a charging roller  120  as charging means which comes in contact with the peripheral surface of the photoconductive drum  110  and uniformly charges the peripheral surface, and cleaning means  130 . 
     The developing unit  200  is provided with a developing section  210 Y for yellow, a developing section  210 C for cyan, a developing section  210 M for magenta, and a developing section  210 K for black as developing means. These developing sections  210 Y,  210 C,  210 M and  210 K respectively contain toner of yellow, cyan, magenta and black. The above developing sections are respectively provided with developing rollers  211 Y,  211 C,  211 M and  211 K, and are set so that only one of the above developing sections can come in contact with the photoconductive drum  110  at a time. 
     The intermediate transfer unit  300  is provided with a driving roller  310 , a primary transfer roller  320 , a wrinkle removing roller  330 , a tension roller  340 , a backup roller  350 , an intermediate transfer belt  360  having no end and being extended around each roller, and cleaning means  370  touchable to or detachable from the intermediate transfer belt  360 . 
     A secondary transfer roller  380  is arranged opposite to the backup roller  350 . The secondary transfer roller  380  is supported so that the secondary transfer roller can be turned by an arm  382  supported by a supporting shaft  381  so that the arm can be swung. The secondary transfer roller is touched to or detached from the intermediate transfer belt  360  when the arm  382  is swung by the operation of a cam  383 . 
     A gear  311  shown in FIG. 5 is fixed to the end of the driving roller  310 , and is rotated at approximately the same peripheral speed as the photoconductive drum  110 , because the gear  311  is engaged with a gear  144  (see FIG. 5) of the photoconductor unit  100 . Therefore, the intermediate transfer belt  360  is circulated at approximately the same peripheral speed as the photoconductive drum  110 . 
     In a process in which the intermediate transfer belt  360  is circulated, a toner image on the photoconductive drum  110  is transferred on the intermediate transfer belt  360  between the primary transfer roller  320  and the photoconductive drum  110 , and the toner image transferred onto the intermediate transfer belt  360  is transferred onto a recording medium S, such as paper, supported between the intermediate transfer belt and the secondary transfer roller  380 . The recording medium S is supported from the paper supply unit  70 . 
     The paper supply unit  70  is provided with a tray  71  on which plural sheets of recording mediums S are piled, a pickup roller  72 , a hopper  73  for pushing the recording mediums S piled on the tray  71  toward the pickup roller  72 , and a pair of separating rollers  74  for securely separating recording mediums fed by the pickup roller  72 . 
     A recording medium S fed by the paper supply unit  70  is supplied to a secondary transfer part, that is, between the intermediate transfer belt  360  and the secondary transfer roller  380  through a pair of first carriage rollers  91 , a first paper sensor  91 S, a pair of second carriage rollers  92 , a second paper sensor  92 S, and a pair of gate rollers  93 , and afterward, ejected onto the case  150  through the fixing unit  400 , a pair of first ejecting rollers  94 , and a pair of second ejecting rollers  95 . 
     The fixing unit  400  is provided with a fixing roller  410  provided with a heat source, and a pressurizing roller  420  pressed on the fixing roller. 
     The operation of the above whole image formation apparatus is as follows: 
     (i) When a printing command signal (an image formation signal) from a host computer (not shown) such as a personal computer is input to the control unit  80 , the photoconductive drum  110 , the developing roller and the like of the developing unit  200 , and the intermediate transfer belt  360  are rotated. 
     (ii) The peripheral surface of the photoconductive drum  110  is uniformed charged by the charging roller  120 . 
     (iii) Selective exposure L according to the image information of a first color (for example, yellow) is applied to the peripheral surface of the uniformly charged photoconductive drum  110  by the exposure unit  60  so that an electrostatic latent image for yellow is formed. 
     (iv) Only the developing roller  211 Y of the developing section  210 Y for the first color (for example, yellow) is touched to the photoconductive drum  110 , hereby, the above electrostatic latent image is developed and the toner image of the first color (for example, yellow) is formed on the photoconductive drum  110 . 
     (v) The toner image formed on the photoconductive drum  110  is transferred onto the intermediate transfer belt  360  in a primary transfer part, that is, between the photoconductive drum  110  and the primary transfer roller  320 . At this time, the cleaning means  370  and the secondary transfer roller  380  are detached from the intermediate transfer belt  360 . 
     (vi) After toner left on the photoconductive drum  110  is removed by the cleaning means  130 , the photoconductive drum  110  is deelectrified by deelectrifying light L′ from deelectrification means. 
     (vii) The operation shown in the above items (ii) to (vi) is repeated if necessary. That is, processing for second, third and fourth colors is repeated according to the contents of the above printing command signal, and a toner image according to the contents of the printing command signal is overlapped on the intermediate transfer belt  360  and is formed on the intermediate transfer belt  360 . 
     (viii) A recording medium S is supplied from the paper supply unit  70  at predetermined timing. Immediately before or after the end of the recording medium S reaches the secondary transfer part (in short, at timing at which a toner image on the intermediate transfer belt  360  is transferred in a desired position on the recording medium S), the secondary transfer roller  380  is pressed to the intermediate transfer belt  360 , and the toner image (basically, a full color image) on the intermediate transfer belt  360  is transferred on the recording medium S. The cleaning means  370  then comes in contact with the intermediate transfer belt  360  and, after secondary transfer, toner left on the intermediate transfer belt  360  is removed. 
     (ix) When the recording medium S passes the fixing unit  400 , a toner image is fixed on the recording medium S and afterward, the recording medium S is ejected on the case  150  via a pair of the paper ejecting rollers  94  and  95 . 
     The outline of the image formation apparatus is described above. Next, the details of the intermediate transfer unit  300  will be described. 
     FIG. 4 is a side view, a part of which is omitted, showing the intermediate transfer unit  300 . 
     As described above, the intermediate transfer unit  300  is provided with the driving roller  310 , the primary transfer roller  320 , the wrinkle removing roller  330 , the tension roller  340 , the backup roller  350 , the intermediate transfer belt  360  having no end and being extended around each of the above rollers , and the cleaning means  370  which can be touched to or detached from the intermediate transfer belt  360 . The above members, and others, are attached to a frame  301  as shown in FIG.  4 . 
     The frame  301  is constituted by a pair of side plates (in FIG. 4, the side plate on this side is omitted), and each of the above members, and others, are attached between both side plates. In other words, the frame is constructed so that a pair of the side plates are coupled by the shafts of the above members. 
     The driving roller  310  is supported on the frame  301  by its shaft  312  so that the driving roller can be rotated, and the above gear  311  shown in FIG. 5 is fixed to the end thereof. The driving roller is constructed so that it is rotated at approximately the same peripheral speed as the photoconductor unit  100  because the gear  311  is engaged with the gear  144  of the photoconductor unit  100 . As shown in FIG. 5, reference number  500  denotes a driving motor. The photoconductive drum  110  is rotated because a pinion  510  fixed to the driving motor output shaft  501  is engaged with the gear  144  provided at an end of the photoconductive drum  110  via a reduction gear  520 . The gear  311  is engaged with the driving gear  133   b  of a toner carriage screw  133  in the photoconductor unit  100  shown in FIG. 3 via an intermediate gear  520  and a reduction gear  521  and hereby, the toner carriage screw  133  is rotated. 
     As shown in FIG. 4, the shaft  321  of the primary transfer roller  320  is supported by the frame  301  via a pair of bearing members  322  so that the primary transfer roller can be rotated. An electrode plate  323  for applying voltage to the primary transfer roller  320  is supported by screwing its long hole  323   a  to a tapped hole  302  provided to the frame  301 . The bearing member  322  is supported by a concave portion  303  provided to the frame  301  so that the bearing member can be slid (can be moved vertically in FIG.  4 ), and a compression coil spring  324  as pressing means is provided between the bearing member  322  and the frame  301 . 
     Therefore, the primary transfer roller  320  is pressed onto the photoconductive drum  110  via the intermediate transfer belt  360  because the both ends of the shaft  321  are respectively pressed by the pair of compression coil springs  324 . 
     The wrinkle removing roller  330  is supported on the frame  301  by its shaft  331  so that the wrinkle removing roller can be rotated. 
     The tension roller  340  is supported so that its shaft  341  can be rotated and slid in a long hole  304  provided in the frame  301 . One end  342   a  of an arm  342  forming a pair at both ends is in contact with the shaft  341 . The arm  342  is supported on the frame  301  by its shaft  343  so that the arm can be swung, and a tension spring  344  is provided between the other end  342   b  and the frame  301 . 
     Therefore, the tension roller  340  is pressed via the arm  342  by the tension spring  344  in a direction in which the intermediate transfer belt  360  is always tensed. 
     The backup roller  350  is supported on the frame  301  by its shaft  351  so that the backup roller can be rotated. 
     The intermediate transfer belt  360  is extended around each roller  310 ,  320 ,  330 ,  340  and  350  and circulated by the driving roller  310  in a direction (clockwise) shown by arrows in FIG.  4 . 
     The cleaning means  370 , disposed within or adjacent to a case  374 , includes a fur brush  371  for brushing toner left and stuck on the peripheral surface of the intermediate transfer belt  360 , a cleaner blade  372  for further scratching toner still left and stuck on the peripheral surface of the intermediate transfer belt  360 , and a toner carriage screw  373  as carriage means for carrying the toner brushed or scratched by the above fur brush  371  or cleaner blade  372 . 
     A toner withdrawal chamber  375  is formed in the lower part of the case  374 , and the above fur brush  371 , cleaner blade  372  and toner carriage screw  373  are arranged in the toner withdrawal chamber  375 . 
     The fur brush  371  is fixed on its shaft  371   a  piercing the side plate of the case  374  and rotated in the direction shown by the arrows in FIG. 4 by the shaft  371   a  being driven by driving means not shown. 
     The cleaner blade  372  is attached to the case  374  via a mounting plate  372   a  and is constructed so that the end (the lower end) comes in contact with the peripheral surface of the intermediate transfer belt  360  and scratches toner. 
     The toner carriage screw  373  is rotated by its shaft  373   a  piercing the side plate of the case  374  being driven by a driving means (not shown), and carries toner collected in the toner withdrawal chamber  375  to a waste toner box (not shown) as waste toner. 
     Cylindrical part  374   a  is provided at both sides of the case  374  is supported on the frame  301  via a bearing member  376  so that the cylindrical part can be rotated. 
     A hook  377  is attached to both sides at the lower end of the case  374 , and a tension spring  378  is provided between the hook-  377  and the frame  301 . 
     Therefore, the case  374  is always biased by the tension spring  378  in a direction (clockwise) in which the fur brush  371  and the cleaner blade  372  press the intermediate transfer belt  360 . However, the turn of the case  374  is regulated by a cam  55  provided for the intermediate transfer unit  300 , as shown in FIG. 3, and is in contact with the lower end of the case  374 . 
     The cam  55  is driven by driving means (not shown). When the cam is located in a position shown in FIG. 4, it turns the case  374  counterclockwise as shown by an alternate long and short dash line, and detaches the fur brush  371  and the cleaner blade  372  from the intermediate transfer belt  360 . 
     In FIG. 4, reference number  156  denotes a position detecting sensor (see FIG. 3) provided on the body of the image formation apparatus so that the position detecting sensor is opposite to the driving roller  310 . The position detecting sensor is provided to detect the position of the intermediate transfer belt  360 . 
     The above intermediate transfer unit  300  is formed so that it can be attached to or detached from the body of the image formation apparatus. 
     Further, in this embodiment, since various contrivances are made or can be made, they will be described below. 
     With respect to driving roller  310   
     (1) The outer diameter of the driving roller  310  is constructed so that the peripheral speed of the intermediate transfer belt  360  is slightly (in a range of tolerance) faster than that of the photoconductive drum  110 . 
     It is desirable that the peripheral speed of the photoconductive drum  110  is completely equal to that of the intermediate transfer belt  360  on which a toner image is transferred from the photoconductive drum  110 . 
     However, since there is tolerance between the outer diameter of the photoconductive drum  110  and that of the driving roller  310 , it is impossible to equalize the above peripheral speeds completely. In such a status, if the peripheral speed of the intermediate transfer belt  360  at a part in which the intermediate transfer belt is wound on the driving roller  310 , is slightly slower than that of the photoconductive drum  110 , a force which tries to loosen the intermediate transfer belt  360  is applied to the intermediate transfer belt  360  between a position (a primary transfer position T 1 ) in which the photoconductive drum  110  and the primary transfer roller  320  arc in contact and the driving roller  310 , though the force is very slight. Thus, a state of the intermediate transfer belt  360  in the primary transfer position TI is made unstable. 
     In this embodiment, the outer diameter of the driving roller  310  is set so that the peripheral speed of the intermediate transfer belt  360  is slightly (in a range of tolerance) faster than that of the photoconductive drum  110 . 
     When the above structure is made, since the intermediate transfer belt  360  between the position (the primary transfer position T 1 ) in which the photoconductive drum  110  and the primary transfer roller  320  are in contact and the driving roller  310  is always tensed, though the tensed quantity is slight, the state of the intermediate transfer belt  360  in the primary transfer position T 1  is stabilized. 
     The deflective quantity of the peripheral surface of the driving roller  310  is set to ±0.05 mm or less. 
     (2) The intermediate transfer belt  360  is constructed so that the period is equivalent to the integer-fold period of the driving roller  310 . 
     The quantity of dislocation caused by the deflection of the shaft or peripheral surface of the driving roller  310  between/among toner images of each color overlapped on the intermediate transfer belt  360  can be reduced, as described above. 
     Concretely, the above ratio is set to S to 1. 
     (3) The intermediate transfer belt  360  is constructed so that the period is equivalent to the integer-fold period of the photoconductive drum  110 . 
     The quantity of dislocation caused by the deflection of the shaft or peripheral surface of the photoconductive drum  110  between/among toner images of each color overlapped on the intermediate transfer belt  360  can be reduced, as described above. 
     Concretely, the above ratio is set to 2 to 1. 
     (4) The angle of the contact of the intermediate transfer belt  360  with the driving roller  310  is set to 90° or more so that the angle of the contact is larger than the angle of the contact with the other roller. 
     The intermediate transfer belt  360  can be stably driven by the above construction even if a friction coefficient between the driving roller  310  and the intermediate transfer belt  360  is small or the friction coefficient is reduced because of long-term use. 
     Concretely, the above angle of the contact is set to approximately 151°. 
     To increase the above friction coefficient, urethane coating is applied to the peripheral surface of the driving roller  310 . 
     With respect to backup roller  350   
     For a method of separating the intermediate transfer belt  360  and a recording medium S at a part in which the backup roller  350  and the secondary transfer roller  380  are in contact, that is, a secondary transfer part T 2  shown in FIG. 4, a curvature separating method is adopted. The diameter of the backup roller  350  is set to 35 mm or less, and the angle of the contact of the intermediate transfer belt  360  with the backup roller  350  is set to 90° or more. 
     A recording medium S is securely separated from the intermediate transfer belt  360  by the above construction. 
     It is desirable that the diameter of the backup roller  350  is set to 30 mm or less and the angle of the contact of the intermediate transfer belt  360  with the backup roller  350  is set to 105° or more. Concretely, the above diameter is set to 30 mm and the above angle of the contact is set to 109°. 
     It is desirable that the surface resistivity of the intermediate transfer belt  360  is set to 10 12  Ω or less. 
     With respect to cleaning means  370   
     (1) The tension roller  340  is put closer to the side of the cleaning means  370  in a horizontal direction as compared with the backup roller  350 , and a part of the toner withdrawal chamber  375  is open under a part in which the fur brush  371  and the intermediate transfer belt  360  are in contact. 
     According to the above construction, toner brushed down by the fur brush  371  is readily collected in the toner withdrawal chamber  375 . 
     It is desirable that an angle θ between the intermediate transfer belt  360  and a vertical line V between the tension roller  340  and the backup roller  350 , that is, an angle θ between a common tangent of the tension roller  340  and the backup roller  350  and a vertical line V is set to 10° or more, and it is more preferable that the above angle is set to 15° or more. 
     According to the above construction, toner brushed down by the fur brush  371  is more securely and more readily collected in the toner withdrawal chamber  375 , and toner dropped when the cleaning means  370  is detached from the intermediate transfer belt  360  is also more readily collected in the toner withdrawal chamber  375 . 
     (2) The tension roller  340  also functions as means for receiving the pressure of the cleaning means  370  upon the intermediate transfer belt  360 . 
     The manufacturing cost can be reduced by the above construction. Since another tension roller is not required to be provided and the number of rollers can be reduced, the angle of the contact of the intermediate transfer belt with each roller is increased. 
     With respect to wrinkle removing roller  330   
     The wrinkle removing roller  330  is arranged on the upstream side close to the primary transfer position T 1  in a direction in which the intermediate transfer belt  360  is circulated, and the angle of the contact of the intermediate transfer belt  360  with the wrinkle removing roller  330  is set to 10° or more. 
     A wrinkle formed on the intermediate transfer belt  360  between the tension roller  340  and the wrinkle removing roller  330  (a wavy state when viewed from the wrinkle removing roller  330  to the tension roller  340 ) is removed by the wrinkle removing roller  330 , and the intermediate transfer belt  360  in the primary transfer position T 1  can be smoothed respectively by constituting as described above. 
     It is desirable that the angle of the contact of the intermediate transfer belt  360  with the wrinkle removing roller  330  is set to 15° or more. Concretely, the above angle is set to 17.6°. 
     Means for changing the proceeding direction of the intermediate transfer belt  360  by 100 or more, such as a guide plate, may be provided in place of the wrinkle removing roller  330 . 
     With respect to primary transfer position T 1   
     (1) The driving roller  310 , the primary transfer roller  320  and the wrinkle removing roller  330  arc arranged so that the intermediate transfer belt  360  is straight tensed in a direction of a tangent to the photoconductive drum  110  at the primary transfer position T 1 . 
     A transfer nip can be stabilized without depending upon belt tension by the above construction. If the intermediate transfer belt  360  is wound on the primary transfer roller  320  and the primary transfer position T 1  is formed at the wound part, the variation of the tension of the intermediate transfer belt  360  has a large effect upon the primary transfer position T 1 . However, the above effect can be reduced by placing the intermediate transfer belt  360  under tension in a direction of a tangent to the photoconductive drum  110  without winding the intermediate transfer belt  360  on the primary transfer roller  320 . 
     (2) The primary transfer position T 1  is arranged close to the driving roller  310 . 
     If distance between the primary transfer position T 1  and the driving roller  310  is large, the shrinkage of the intermediate transfer belt  360  between them is increased and the travel speed of the intermediate transfer belt  360  in the primary transfer position T 1  becomes unstable. 
     In this embodiment, the travel speed of the intermediate transfer belt  360  at the primary transfer position T 1  is stabilized by arranging the primary transfer position T 1  close to the driving roller  310 . 
     It is desirable that distance L 1  shown in FIG. 4 between the primary transfer position T 1  and the driving roller  310  is set to 40 mm or less, and it more is preferable that the above distance is set to 35 mm or less. Concretely, the distance is set to approximately 30.5 mm. 
     (3) For the length of the straight part of the intermediate transfer belt  360  from the wrinkle removing roller  330  to the driving roller  310 , the aspect ratio is set to 0.25 or less. It is preferable that it is set to 0.15 or less. 
     Based on the above construction, a wrinkle, and the corresponding effects, can be more effectively inhibited. 
     Concretely, the length of the above straight part is set to approximately 55.5 mm. 
     With respect to positional detection 
     As described above, the position detecting sensor  156  is arranged opposite to the driving roller  310  to detect the position of the intermediate transfer belt  360  on the driving roller  310 . 
     Hereby, the travel cycle of the intermediate transfer belt  360  can be precisely detected. 
     The position detecting sensor  156  is constituted by a reflector type optical sensor and a mark to be detected by the position detecting sensor  156  is provided on the intermediate transfer belt  360  by printing. 
     When the position detecting sensor is constituted by a transmitted light sensor and a hole to be detected by the sensor is made on the intermediate transfer belt  360 , stress is centralized in the hole and the hole is deformed so that precise detection may be impossible. However, in this embodiment, since the position detecting sensor  156  is constituted by a reflector type optical sensor and a mark to be detected by the sensor is provided on the intermediate transfer belt  360  by printing, the travel cycle of the intermediate transfer belt  360  can be precisely detected. 
     With respect to construction in which the intermediate transfer belt  360  is tensed and extended 
     For construction in which the intermediate transfer belt  360  is tensed, the length of the intermediate transfer belt  360  from the primary transfer position T 1  to the secondary transfer position T 2  is set to the length in the transverse direction of A4-sized paper or longer, and the length of the intermediate transfer belt  360  from the secondary transfer position T 2  to the primary transfer position T 1  is also set to the length in the transverse direction of A4-sized paper or longer. That is, the intermediate transfer belt  360  is tensed and extended to realize the length described above. 
     According to the above constriction, when printing on A4-sized paper is continuously executed, timing at which the secondary transfer roller  380  is touched to the intermediate transfer belt  360  can be set in the unit of paper, that is, the secondary transfer roller  380  can be prevented from being touched to the intermediate transfer belt during primary transfer. 
     When the secondary transfer roller  380  is touched to the intermediate transfer belt  360  during primary transfer, an image by primary transfer may be deformed by the shock. However, such a situation can be prevented by the above construction. 
     With respect to cleaning means  370   
     (1) The cleaner blade  372  is made of urethane rubber, the free length is set to approximately 8 mm, the thickness is set to approximately 3 mm, the Young&#39;s modulus is set to approximately 7 to 9 MPa, the holder angle (an angle between the blade in a state of no load and the tangent of the roller in the contact position) is set to approximately 20°, and the contact pressure on the intermediate transfer belt  360  is set to approximately 45 gf/cm. 
     According to the above construction, cleaning failure caused by the passage of toner through the blade or by the vibration and lifting of the blade can be prevented. 
     (2) The waste toner box is provided apart from the case  374 . 
     Since a large quantity of waste toner can be prevented from being collected in the case  374  according to the above construction, the variation of load when the case  374  is swung and force operating on the case  374  after the case is swung, can be reduced. As a result, the contact pressure of the cleaner blade  372  on the intermediate transfer belt  360  can be stabilized. 
     (3) The shaft  373   a  (see FIG. 4) of the toner carriage screw  373  is located in the center of the turning of the case. 
     According to the above construction, relative positional relationship between the case and the other fixed member, for example between the waste toner carriage port of the case  374  and the toner receiving port of the waste toner box is readily secured. 
     (4) The cam  155  is constituted by a SIN cam. 
     Shock applied to the intermediate transfer belt  360  can be reduced by the above construction. 
     With respect to patch sensing 
     Patch sensing, that is, the detection of toner quantity in trial printing is executed on the intermediate transfer belt  360  on the driving roller  310 . 
     The above patch sensing can be executed at a place in which the angle of contact is large and speed is stable by the above construction. 
     With respect to bead 
     A bead is a bump provided on the rear of the intermediate transfer belt  360  along the circulated direction and the position (in the direction of the axis of each roller) of the belt is regulated by fitting the beads into a concave groove (a regulating groove) formed on each roller on which the belt is wound. 
     The above beads are not necessarily provided and in the embodiment shown in FIG. 4, they are also not provided. If they are provided, they are to be constructed as follows: 
     (1) Silicon rubber is used for the bead, the thickness (the height of protrusion) is set to approximately 1.5 mm, and the width is set to approximately 4 mm. 
     (2) The coefficient of friction between the bead and the regulating groove is set so that it is smaller than that between the base material of the intermediate transfer belt  360  and any roller. 
     The occurrence of a tension inclination in the axial direction of the belt by frictional force between the bead and the regulating groove can be reduced by constructing as described above. 
     The coefficient of friction between the base material of the intermediate transfer belt  360  and any roller is approximately 0.4. 
     (3) The elastic strength of the bead is set to approximately 2.0 to 8.0 MPa. 
     If the bead is too soft, stress against thrust in a regulating part is applied to only one place, that is, a small range in which the bead is bonded. 
     On the contrary, if the bead is too hard, the effect of the bead upon the bent part of the belt is too large. 
     It is desirable that the elastic strength of the bead is set to {1.0 to (t 1 /t 2 ) 2 } E 1  [MPa], where t 1  means the thickness of the belt, t 2  means the thickness of the bead, and E 1  means Young&#39;s modulus (up to 4.0×10 3  MPa) of the belt. 
     (4) The bead regulating groove is provided to each roller which is not adjacent to the primary transfer position T 1 . 
     According to the above construction, dislocation between/among toner images of each color overlapped on the intermediate U-transfer belt  360  can be reduced by the random variation by contact between the bead and the regulating groove of the intermediate transfer belt  360 . 
     For example, the bead regulating groove is constructed by attaching a stepped flange to the end of the backup roller  350 . 
     (5) The regulating groove is formed so that the width is slightly larger than that of the bead and the regulating groove has a margin for the straightness of adhesion of the bead. 
     For example, if the width of the bead is approximately 4 mm, that of the regulating groove is set to approximately 4.2 mm. 
     With respect to replacement and handling of intermediate transfer unit  300   
     (1) The intermediate transfer unit  300  is constructed so that the intermediate transfer belt  360  does not come in contact with the surface of a desk and others when the intermediate transfer unit  300  is put on the desk. Thus, the intermediate transfer belt  360  is prevented from being damaged or a foreign matter is prevented from adhering onto the intermediate transfer belt. 
     (2) The intermediate transfer unit  300  is constructed so that a drive transmission part such as the gear  311  does not come in contact with the surface of a desk when the intermediate transfer unit  300  is put on the desk. Thus, the deformation and damage of the drive transmission part are prevented. 
     (3) The electrode part of the intermediate transfer unit  300  is provided on the reverse side of the drive transmission part, so that an electrode is prevented from being stained and the failure of a contact is prevented. 
     (4) The intermediate transfer unit  300  is constructed so that the photoconductor unit  100  cannot be installed when the intermediate transfer unit  300  is not installed. Thus, erroneous attachment is prevented. 
     (5) The intermediate transfer unit  300  is constructed so that the capacity of the waste toner box is related to the life of the intermediate transfer belt  360  and the waste toner box is also replaced when the intermediate transfer unit  300  is replaced. Thus, the handling is enhanced. 
     With respect to sequence 
     (1) When the position of the intermediate transfer belt  360  as the basis of exposure writing timing is detected, bias for primary transfer is applied, that is, bias for primary transfer is applied before detecting the position. 
     The load of each color onto the intermediate transfer belt  360  in the primary transfer position T 1  from the detection of the position to primary transfer is approximately equal, and dislocation (called misregistration) among toner images of each color overlapped on the intermediate transfer belt  360  can be inhibited, as described above. 
     (2) The position of the mark for detecting the position when the intermediate transfer belt  360  is stopped is set so that it is located on the upstream side of the primary transfer position T 1 . For example, the above position on the upstream side is a position shown by M in FIG.  4 . 
     Since the position is detected when the tension of the intermediate transfer belt  360  is stable because of the application of bias in the initial circulation of the intermediate transfer belt  360 , misregistration caused by the dislocation of the period can be avoided by setting as described above. 
     With respect to frame  301  of intermediate transfer unit  300   
     The side plate of the frame  301  is constituted by an insulating member so that the insulation to a roller shaft for applying bias to a roller (and/or a bearing member) is not required. 
     The coefficient of the thermal expansion of the frame  301  is approximately equalized to that of the intermediate transfer belt  360  by using acrylonitrile butadiene styrene resin (ABS resin) as the above insulating member, and relative misregistration due to the change of temperature can be prevented. 
     Embodiments 
     Further concrete embodiments will be described below. 
     The following description is mainly related to a transfer process: 
     For stabilizing the efficiency of primary transfer 
     (1) A high-voltage power source which has constant-current control when the impedance of primary transfer is large (approximately 30 MΩ or more) and has constant-voltage control when the impedance is small (approximately 30 MΩ or less), is used. 
     Therefore, even if there is dispersion in the quantity (film thickness) of toner, environment, and the resistance of a member, transfer is satisfactorily executed. 
     (2) The surface resistivity of the intermediate transfer belt  360  is set to 10 8  to 10 12  Ω and the volume resistivity is set to 10 8  to 10 12  Ωcm. 
     The primary transfer roller  320  is made of urethane in which carbon is dispersed, the resistance thereof is set to 10 6  to 10 8  Ω (desirably approximately 10 7  Ω), the hardness is set to 45±5°, and the load onto the photoconductive drum  110  by the primary transfer roller is set to 1.0 to 3.5 kg (desirably approximately 2.5 kg). 
     Transfer is enabled at 1200 V or less by setting the resistance value to the above range. 
     The occurrence of a so-called void can be prevented by setting the hardness and the load to the above range. 
     3) For the quantity of a used additive to toner, the quantity of an additive with a large particle diameter is set to 0.5 to 4.0 wt % (desirably approximately 0.7 wt %) and the quantity-of an additive with a small particle diameter is set to 1.5 to 4.0 wt % (desirably approximately 2.0 wt %). 
     The additive with a large particle diameter is mainly required to enhance the stability of the durability of toner, and in view of the above, the more the quantity of the above additive is, the better the result. However, if the quantity of the above additive exceeds 4.0 wt %, the fluidity of toner is deteriorated, and the occurrence of a void and the like may be caused. Thus, too much of the above additive is not desirable. 
     In the meantime, the additive with a small particle diameter is mainly required to enhance transferability on rough paper, and in view of the above, the more the quantity of the above additive is, the better the result. However, if the quantity of the above additive exceeds 4.0 wt %, the photoconductive drum  110  and the intermediate transfer belt  360  are readily filmed with floating silica. Thus, too much of the above additive is not desirable. 
     The deterioration of an image due to interference in simultaneous primary and secondary transfer can be prevented and the capacity of the high-voltage power source can be reduced to a minimum under the conditions described in above (1) to (3). 
     For stabilization of secondary transfer efficiency 
     (1) A high-voltage power source which has constant-current control when the impedance of secondary transfer is large (approximately 20 MΩ or more) and has constant-voltage control when the impedance is small (approximately 20 MΩ or less), is used. 
     Hereby, even if there is dispersion in the type of paper, environment, and the resistance of a member, transfer is satisfactorily executed. 
     (2) The surface resistivity of the intermediate transfer belt  360  is set to 10 8  to 10 12  Ω and the volume resistivity is set to 10 8  to 10 12  Ωcm. 
     The secondary transfer roller  380  is an ionic roller, the resistance thereof is set to 10 6  to 10 8  Ω, the hardness is set to 60±5°, and the load onto, the backup roller  350  by the secondary transfer roller is set to 5.0 to 9.0 kg (desirably approximately 7.0 kg). 
     Transfer is enabled at 4000 V or less and 200 μA or less by setting the resistance to the above range. 
     The backup roller  350  is grounded. 
     (3) For the quantity of a used additive to toner, the quantity of an additive with a large particle diameter is set to 0.5 to 4.0 wt % (desirably approximately 0.7 wt %) and the quantity of an additive with a small particle diameter is set to 1.5 to 4.0 wt % (desirably approximately 2.0 wt %). 
     The reason is as described above. 
     For preventing the rear of recording medium S such as paper from being stained 
     When transfer on paper or the transfer of a color is not executed while the secondary transfer roller  380  is in contact with the intermediate transfer belt  360 , voltage approximately 0 to −600 V in a direction in which toner is returned to the intermediate transfer belt  360 , is applied. 
     Toner which adheres to the secondary transfer roller  380  is reduced and a stain on the rear of a recording medium S is reduced by the above construction. 
     For satisfactorily transferring on rough (bond) paper 
     (1) The hardness of the secondary transfer roller  380  is set to 60±5° and the load onto the backup roller  350  by the secondary transfer roller is set to 5.0 to 9.0 kg (desirably approximately 7.0 kg). 
     (2) For the quantity of a used additive to toner, the quantity of an additive with a large particle diameter is set to 0.5 to 4.0 wt % (desirably approximately 0.7 wt %) and the quantity of an additive with a small particle diameter is set to 1.5 to 4.0 wt % (desirably approximately 2.0 wt %). 
     For toner, high density pigment toner with the particle diameter of approximately 7 μm is used. 
     (3) The quantity of toner before secondary transfer, that is, the quantity of toner on the intermediate transfer belt  360  is set to 1.5 mg/cm 2  or less. 
     A satisfactory transfer state can be also acquired on rough paper such as bond paper by setting as described in above (1) to (3). 
     That is, the surface of paper can be touched closely to toner by setting-the hardness of the secondary transfer roller  380  to a high value as described above and setting a load onto the secondary transfer roller to a high value. Thus, even if a high electric field is formed, the failure of transfer due to discharge is reduced. A state in which paper is carried is also stabilized by applying the high load. 
     Further, the transfer efficiency of toner can be enhanced by reducing the quantity of toner before secondary transfer as described above. 
     For preventing the occurrence of a void 
     (1) The intermediate transfer belt  360  is made of ethylene tetrafluoroethylene (ETFE) in which carbon black and others are dispersed as a conductor, polyethylene terephthalate (PET) generated by depositing aluminum and further coating with urethane paint including fluoric particulates, or polyimide in which carbon black and others are dispersed as a conductor. 
     The photoconductive drum  110  is made of polycarbonate. 
     (2) The hardness of the primary transfer roller  320  is set to 45±5° and the load onto the photoconductive drum  110  by the primary transfer roller is set to 1.0 to 3.5 kg. 
     (3) The hardness of the secondary transfer roller  380  is set to 60±5° and the load onto the backup roller  350  by the secondary transfer roller is set to 5.0 to 9.0 kg. 
     (4) For the quantity of a used additive to toner, the quantity of an additive with a large particle diameter is set to 0.5 to 4.0 wt % (desirably approximately 0.7 wt %) and the quantity of an additive with a small particle diameter is set to 1.5 to 4.0 wt % (desirably approximately 2.0 wt %). 
     The fluidity of toner is set to approximately 0.35 g/cc. 
     The following function and effect can be acquired by setting as described above: 
     That is, as for the condition of transfer from the photoconductive drum  110  to the intermediate transfer belt  360  in the primary transfer part, the low hardness, the low load and the high fluidity of toner is used, so that the occurrence of a void is prevented. 
     For the condition of transfer from the intermediate transfer belt  360  in the secondary transfer part, the high hardness and the high load of toner is used. However, since the intermediate transfer belt  360  is made of fluorine and toner is very fluid, the occurrence of a void is prevented. 
     For reducing the scattering of toner 
     (1) The wrinkle removing roller  330  is provided close on the upstream side of the primary transfer position T 1 . 
     (2) For the quantity of a used additive to toner, the quantity of an additive with a large particle diameter is set to 0.5 to 4.0 wt % (desirably approximately 0.7 wt %) and the quantity of an additive with a small particle diameter is set to 1.5 to 4.0 wt % (desirably approximately 2.0 wt %). 
     The fluidity of toner is set to approximately 0.35 g/cc and the quantity of electrostatic charge is set to −10 μC/g or more. 
     (3) The surface roughness of the intermediate transfer belt  360  is set to Rmax 1 μm (desirably 0.7 μm) or less. 
     The surface resistivity of the intermediate transfer belt  360  is set to 10 8  to 10 12  Ω, and the volume resistivity is set to 10 8  to 10 12  Ωcm. 
     The following function and effect can be acquired by the setting, as described above: 
     In the primary transfer part, wrinkles of the intermediate transfer belt  360  are reduced by the wrinkle removing roller  330  and scattering is reduced. 
     In the secondary transfer part, toner on the intermediate transfer belt  360  is stably carried and scattering is reduced. 
     For the reduction of the cost 
     (1) The intermediate transfer belt  360  without an end is formed by coating a sheet-shaped PET on which aluminum is deposited, with urethane paint in which PEFT particles and SnO as a conductor are dispersed, and by bonding both ends through ultrasonic welding. 
     Difference in a level made by bonding both ends is set to 50 μm or less and desirably set to 30 μm or less. Young&#39;s modulus of the paint is set to approximately 1.5×10 4  kgf/cm 2 . The surface resistivity of the paint is set to approximately 10 8  to 10 12  Ω and the surface roughness is set to Rmax 1 μm (desirably 0.7 μm) or less. As for the construction of an electrode, a conductive layer is printed on the surface of aluminum at an end, and bias is applied by a roller electrode (1 MΩ or less). 
     (2) The high-voltage power source has current absorption type constant-voltage control in the primary transfer part, and applies primary transfer voltage until secondary transfer is finished. 
     The efficiency of transfer and the property of cleaning can be enhanced by setting as described in above (1) and (2). 
     The primary transfer roller functions only as the backup roller and it is not required to fulfill the function as an electrode. 
     Further, the deterioration of an image due to interference in simultaneous primary and secondary transfer can be avoided by constructing the electrode and the power source as described above. 
     As described above, according to the intermediate transfer unit, the shrinkage of the intermediate transfer belt between the primary transfer position and the driving roller is reduced, so that the travel speed of the intermediate transfer belt in the primary transfer position is stable and as a result, primary transfer in a satisfactory state can be readily acquired. 
     Although the embodiments or examples of the present invention are described above, the present invention is not limited to the above embodiments or examples and may be suitably varied in the range of the gist of the present invention. 
     For example, the following modifications are possible. 
     For satisfactorily transferring on rough paper (bond paper) 
     (1) The outer diameter of the elastic body of the secondary transfer roller  380  is set to 25 mm, the outer diameter of the shaft is set to 15 mm, the length of the elastic body in the direction of the shaft is set to 332 mm, the hardness of the secondary transfer roller is set to 60±10° (desirably approximately 60±5°), and the load onto the backup roller  350  by the secondary transfer roller is set to 5.0 to 9.0 kg (or 15 gh/mm to 27 gf/mm), and desirably to approximately 7.0 kg (or approximately 21 gf/mm). 
     (2) For the quantity of a used additive to toner, the quantity of an additive with a large particle diameter is set to 0.5 to 4.0 wt % (desirably approximately 0.7 wt %) and the quantity of an additive with a small particle diameter is set to 1.5 to 4.0 wt % (desirably approximately 2.0 wt %). The surface coverage can be calculated according to the following expression 1, and the surface coverage for toner with a mother particle diameter of 7 μm in which silica with a particle diameter of 40 nm is added by 0.7 wt % and silica with a particle diameter of 9 nm is added by 2.0 wt %, is 2.8.                                                              γ   =       ∑     i   =   1     n          (       1   π          R     r   i            ρ     ρ   i              W   i     100       )                   Surface                 coverage                     R   :     Outer                 diameter                 of                 toner                 mother                 particle                       ri   :     Outer                 diameter                 of                 additive                 i                       ρ   :     Density                 of                 toner                 mother                 particle                       ρ                   i   :     Density                 of                   additive   —        i                           Wi   :     Quantity                   (     wt                 %     )                   of                 additive                 i                 added                 to                                           toner                 mother                 particle                       i              :       ‘   i   ’        th                 additive                       n   :     Number                 of                 types                 of                 additives                   Expression                 1                         
     (3) The quantity of toner before secondary transfer, that is, the quantity of toner on the intermediate transfer belt  360  is set to 1.5 mg/cm 2  or less. 
     A satisfactory transfer state can be also acquired on rough paper such as bond paper, the surface of which is a rough, of recording medium by setting as described in above (1) to (3). 
     That is, if the linear pressure of the secondary transfer roller  380  is set to 20 gf/mm or more, a sufficient electric field can be formed in a toner layer by adjusting a concave portion of rough (bond) paper to a toner image on the intermediate transfer belt  360  and bringing the concave portion close to the toner image, and the failure of transfer due to discharge in a high electric field is reduced. Further, when the hardness of the secondary transfer roller  380  is set to 50° or more in case the hardness is measured by an Asker-C hardness meter, no increase of torque by excessive nip width occurs and a state in which paper is carried is also stabilized by a stable nip. 
     Further, since the fluidity of toner is secured and the adhesive strength to the intermediate transfer belt can be reduced by adding an additive with a small particle diameter so that the surface coverage of the additive for toner is 2.0 or more, the efficiency of transfer on rough paper can be enhanced. Further, an additive is hardly embedded in a toner mother particle or hardly peeled in long-term use by adding the additive with a large particle diameter as described above, and the enhancement of the durability and transferability on rough paper are compatible. 
     Further, the transfer efficiency of toner can be enhanced by reducing the quantity of toner before secondary transfer as described above. That is, if a primary transfer image consisting of overlapped two layers of solid images on the photoconductive drum is transferred on rough paper, potential difference to be applied between the surface of the intermediate transfer medium and the surface of a recording medium can be reduced and the failure of transfer due to discharge can be avoided by setting the total quantity of toner in the primary transfer image to 1.5 mg/cm 2  or less. 
     For preventing the occurrence of a void 
     (1) The intermediate transfer belt  360  is made of ethylene tetrafluoroethylene (ETFE) in which carbon black and others are dispersed as a conductor, polyethylene terephthalate (PET) generated by depositing aluminum and further coating with urethane paint including fluoric particulates, or polyimide in which carbon black and others are dispersed as a conductor. 
     The photoconductive drum  110  is made of polycarbonate. 
     (2) The outer diameter of the elastic body of the primary transfer roller  320  is set to 22 mm, the outer diameter of the shaft is set to 12 mm, the length of the elastic body in the direction of the shaft is set to 358 mm, the hardness of the primary transfer roller  320  is set to 45±5°, and the load onto the photoconductive drum  110  by the primary transfer roller is set to 1.0 to 3.5 kg. 
     (3) The outer diameter of the elastic body of the secondary transfer roller  380  is set to 25 mm, the outer diameter of the shaft is set to 15 mm, the length of the elastic body in the direction of the shaft is set to 332 mm, the hardness of the secondary transfer roller  380  is set to 60±10° (desirably approximately 60±5°), and the load onto the backup roller  350  by the secondary transfer roller is set to 5.0 to 9.0 kg (or 15 gf/mm to 27 gf/mm), and desirably to approximately 7.0 kg (or approximately 21 gf/mm). 
     (4) For the quantity of a used additive to toner, the quantity of an additive with a large particle diameter is set to 0.5 to 4.0 wt % (desirably approximately 0.7 wt %) and the quantity of an additive with a small particle diameter is set to 1.5 to 4.0 wt % (desirably approximately 2.0 wt %). The surface coverage can be calculated according to the expression 1, and the surface coverage of the additive for toner with a mother particle diameter of 7 μm in which silica with a particle diameter of 40 nm is added by 0.7 wt % and silica with a particle diameter of 9 nm is added by 2.0 wt %, is 2.8. 
     The fluidity of toner is set to approximately 0.35 g/cc. 
     By setting as in above (1) to (3), a satisfactory transfer state can be also acquired on a recording medium such as OHP the surface of which is smooth. 
     That is, as for the condition of transfer from the photoconductive drum  110  to the intermediate transfer belt  360  in the primary transfer part, the low hardness, the low load and the high fluidity of toner is used, so that the occurrence of a void is prevented. 
     For the condition of transfer from the intermediate transfer belt  360  in the secondary transfer part, the high hardness and the high load of toner is used. However, since the intermediate transfer belt  360  is made of fluorine and can be readily released from a mold, the occurrence of a void is prevented. 
     Further, since the concentration of transfer pressure upon a linear image on the intermediate transfer belt  360  is avoided because the hardness of the secondary transfer roller is set to 70° or less in case the hardness is measured by Asker-C hardness meter, the occurrence of a void is prevented. 
     Further, since the fluidity of toner is secured and the adhesive strength to the intermediate transfer belt can be reduced by adding an additive with a small particle diameter so that the surface coverage of the additive for toner is 2.0 or more, the occurrence of a void is prevented. Further, an additive is hardly embedded in a toner mother particle or hardly peeled in long-term use by adding the additive with a large particle diameter as described above, and the enhancement of the durability and transferability on rough paper are compatible. 
     Further, since the height of a toner layer is limited by reducing the quantity of toner before secondary transfer as described above, pressure upon toner is equalized and the occurrence of a void is prevented. 
     For preventing the rear of recording medium S such as paper from being stained 
     When the secondary transfer roller  380  is directly touched to the intermediate transfer belt  360 , an electric field in a direction in which toner is returned from the secondary transfer roller  380  to the intermediate transfer belt  360  (for example, the voltage of approximately 0 to −600 V) is applied to the secondary transfer roller  380 , and when the joint of the intermediate transfer belt  360  is located in the secondary transfer position T 2 , the secondary transfer roller  380  is detached. 
     Toner which adheres to the secondary transfer roller  380  is reduced and a stain which adheres to the rear of a recording medium S is reduced by the above construction. That is, although toner which cannot be removed by the cleaning means  370  is left in a step portion of the joint of the intermediate transfer belt  360 , since the secondary transfer roller  380  is not directly touched to the portion and the secondary transfer roller  380  can be cleaned at another part by bias, a stain by toner on the secondary transfer roller  380  can be reduced and hereby, a stain on the rear of a recording medium can be reduced. 
     Further, according to the intermediate transfer unit of the invention, it is possible to prevent a phenomenon in which toner adheres to the secondary transfer roller by directly touching the secondary transfer roller to the joint of the intermediate transfer medium, and therefore, the rear of a recording medium will not be stained, and the intermediate transfer unit for enabling satisfactory transfer can be readily obtained. 
     Further, according to the intermediate transfer unit of the invention, since the intermediate transfer belt has excellent mold releasing properties, toner is readily released in secondary transfer. Further, since the hardness of the secondary transfer roller is set to 70° or less, as measured by an Asker-C hardness meter, the concentration of transfer pressure upon a linear image on the intermediate transfer belt  360  can be avoided and as a result, when a thin line image is transferred on a recording medium the surface of which is smooth, the occurrence of a so-called void can be reduced. 
     Further, according to the intermediate transfer unit of the invention, since toner is covered with sufficient quantity of additives, the force of toner which adheres to the intermediate transfer belt can be reduced. Further, since a recording medium the surface of which is rough is pressed on the intermediate transfer belt under sufficient linear pressure, a concave portion of the recording medium can be brought close to a toner image on the intermediate transfer belt and as a result, a satisfactory transfer state can be also acquired for rough paper such as bond paper which is a recording medium the surface of which is rough. 
     The present invention may be further modified as follows. 
     For stabilizing the efficiency of primary transfer 
     (1) A high-voltage power source which has constant-current control when the impedance of primary transfer is large (approximately 30 MΩ or more) and has constant-voltage control when the impedance is small (approximately 30 MΩ or less) is used. 
     Hereby, even if there is dispersion in the quantity (film thickness) of toner, environment, and the resistance of a member, transfer is satisfactorily executed. 
     (2) The surface resistivity of the intermediate transfer belt  360  is set to 10 8  to 10 12  Ω, and the volume resistivity is set to 10 8  to 10 12  Ωcm. 
     The primary transfer roller  320  is a roller with the diameter of 22 mm in which an elastic layer made of urethane resin in which carbon is dispersed, is formed on the peripheral surface of a metallic shaft with the diameter of 12 mm. The resistance of the roller is set to 10 6  to 10 8  Ω (desirably approximately 10 7  Ω), the hardness is set to 45±5°, and the load onto the photoconductive drum  110  by the primary transfer roller is set to 1.0 to 3.5 kg (desirably approximately 2.5 kg). 
     Transfer is enabled at 1200 V or less by setting the resistance value to the above range. 
     The occurrence of a so-called void can be prevented by setting the hardness and the load to the above range. 
     Hardness is measured by an Asker-C hardness meter known to a skilled person. Such a hardness meter is called an indentation hardness meter and it is to be noted that the thickness of an elastic layer has an effect upon the value of hardness measured by such a hardness meter. Hardness described in the present invention does not denote the result of measuring the hardness of an elastic body itself constituting an elastic layer but denotes the result of measurement in a state in which an elastic layer is formed on a roller. 
     (3) For the quantity of a used additive to toner, the quantity of an additive with a large particle diameter is set to 0.5 to 4.0 wt % (desirably approximately 0.7 wt %) and the quantity of an additive with a small particle diameter is set to 1.5 to 4.0 wt % (desirably approximately 2.0 wt %). 
     The additive with a large particle diameter is mainly required to enhance the stability of the durability of toner, and in view of the above, the more the quantity of the above additive is, the better the result. However, if the quantity of the above additive exceeds 4.0 wt %, the fluidity of the toner deteriorates. That is, too much of the above additive causes the occurrence of a void, and other problems, and is not desirable. 
     In the meantime, the additive with a small particle diameter is mainly required to enhance transferability on rough paper, and in view of the above, the more the quantity of the above additive is, the better the result. However, if the quantity of the above additive exceeds 4.0 wt %, the photoconductive drum  110  and the intermediate transfer belt  360  are readily filmed with floating silica, which is not desirable. 
     The deterioration of an image due to interference in simultaneous primary and secondary transfer can be prevented and the capacity of the high-voltage power source can be reduced to the minimum under the conditions described in above (1) to (3). 
     (4) The particle diameter of toner is set to 9 μm or less. 
     It is because if the particle diameter is 9 μm or more, the resolution is deteriorated. 
     FIGS.  6 ( a ) to  6 ( c ) show the particle size distribution of toner used this time. The particle size distribution of the above toner is measured using a coal-tar counter model TA-II. The aperture is 100 μm and for an electrolyte, ISOTON-II is used. 
     In-a table shown in FIG.  6 ( a ), the number is shown in the right field, the volume is shown in the left field, the result of measurement is shown in the lower column, and a value calculated based upon the result of the measurement is shown in the upper column. However, the above volume means volume in case a measured toner particle is regarded as a sphere. 
     In graphs shown in FIGS.  6 ( b ) and  6 ( c ), a bar graph shows numeral data and a linked line graph shows cumulative data. 
     In the table shown in FIG.  6 ( a ), the meaning of each item showing the result of measurement in the lower column is as follows: 
     DIF N: Most basic data and shows numeral data (data showing number of toner) input from I/O device every channel. 
     DIF %: Shows above numeral data (DIF N) every channel by %. 
     CUM N: Shows data acquired by accumulating above numeral data (DIF N). 
     CUM %: Shows data acquired by accumulating above DIF %. 
     The meaning of each item showing a calculated value in the upper column is as follows: 
     25.4μ↑: Shows cumulative % value of 25.4 μm or more. 
     6.35μ↓: Shows cumulative % value of 6.35 μm or less. 
     KURTOSIS: Shows kurtosis of distribution. An image which is satisfactory in transferability and the resolution of which is never deteriorated, can be acquired by setting the particle size distribution in volume to 0.8 or more and setting the particle size distribution in number to 0.3 or more. 
     SKEWNESS: Shows skewness of distribution. An image which is satisfactory in transferability and the resolution of which is never deteriorated, can be acquired by setting the skewness to 0.6 or less in an absolute value in the particle size distribution in volume, and setting the skewness to 0.1 or less in an absolute value in the particle size distribution in number. 
     MEAN: Shows arithmetic means particle size. 
     25%: Shows value of particle size when cumulative reaches 25%. (see the graphs shown in FIGS.  6 ( b ) and  6 ( c ).) 
     50%: Shows value of particle size when cumulative % reaches 50%. (see the graphs shown in FIGS.  6 ( b ) and  6 ( c ).) 
     75%: Shows value of particle size when cumulative reaches 75%. (see the graphs shown in FIGS.  6 ( b ) and  6 ( c ).) 
     CV %: Coefficient (%) of variation An image which is satisfactory in transferability and the resolution of which is never deteriorated, can be acquired by setting both particle size distribution in volume and particle size distribution in number to 28% or less. 
     SDμ: Standard deviation (μm) 
     (5) Shape of toner 
     As for the shape factor of toner, 100 pieces of toner images magnified up to 500 magnifications are sampled at random using FE-SEM (S-800) manufactured by Hitachi, Ltd. for example, the image information is analyzed via an interface by an image analyzer Luzex III by Nireco, Ltd. for example, and values calculated according to the following expressions arc defined as a shape factor. 
     
       
         Shape factor (SF-1)=( MXLNG ) 2   /AREA×II/ 4×100 
       
     
     
       
         Shape factor (SF-2)=( PERI ) 2   /AREA× ¼ II ×100 
       
     
     In the above expressions, MXLNG means the absolute maximum length of toner, PERI means the peripheral length of toner, and AREA means the projected area of toner. 
     The shape factor SF- 1  shows the degree of the roundness of toner, and the shape factor SF- 2  shows the degree of the irregularity of toner. It is desirable that the shape factor SF- 1  of toner is 100 to 150, and it is more preferable that SF- 1  is 100 to 130. It is desirable that the shape factor SF- 2  of toner is 100 to 140, and it is more preferable that SF- 2  is 100 to 125. Transfer efficiency in primary and secondary transfer is enhanced by setting the shape factors SF- 1  and SF- 2  as described above. 
     In the embodiment of the present invention, since primary or secondary transfer means which functions as a transfer electrode for applying transfer voltage to a transfer position, is in contact with each transfer position even if toner with the high fluidity of A.D 0.35 g/cc or more is used, a transfer electric field in each transfer position can be concentrated upon the transfer position. Further, transfer means is pressed in each transfer position, and toner the shape of which is approximately spherical and the surface of which is smooth, is used. Thus, a toner image can be readily compressed in the direction of the height in a transfer position so that cohesion among toner is enhanced. As a result, transfer efficiency is enhanced and simultaneously, the occurrence of a void can be better prevented. The turbulence of a toner image due to mechanical force caused by slight difference in speed between the photoconductive drum or a recording medium and the intermediate transfer belt in a transfer position and others, can be also satisfactorily prevented. 
     There is also an effect that, since a toner image can be readily compressed in the direction of the height without causing the turbulence of an image, the melting of each toner is accelerated and an image satisfactory in coloring and transparency can be formed when a toner image is fixed on a recording medium. 
     For the stabilization of secondary transfer efficiency 
     (1) A high-voltage power source which has constant-current control when the impedance of secondary transfer is large (approximately 20 MΩ or more) and has constant-voltage control when the impedance is small (approximately 20 MΩ or less), is used. 
     Hereby, even if there is dispersion in the type of paper, environment, and the resistance of a member, transfer is satisfactorily executed. 
     (2) The surface resistivity of the intermediate transfer belt  360  is set to 10 8  to 10 12  Ω, and the volume resistivity is set to 10 8  to 10 12  Ωcm. 
     The secondary transfer roller  380  is a roller 25 mm in diameter in which an elastic layer formed by dispersing or melting ion conductive material such as lithium perchlorate in urethane resin, is formed on the peripheral surface of the metallic shaft 15 mm in diameter. The resistance of the roller is set to 10 6  to 10 8  Ω, the hardness is set to 60±5°, and the load onto the backup roller  350  by the secondary transfer roller is set to 5.0 to 9.0 kg (desirably approximately 7.0 kg). 
     Transfer is enabled at 4000 V or less and 200 μA or less by setting the resistance to the above range. 
     Hardness is measured by an Asker-C hardness meter known to a skilled person, and as described above, hardness described in the present invention dose not denote the result of measuring an elastic body itself constituting an elastic layer but denotes the result of measurement in a state in which an elastic layer is formed into a roller. 
     The backup roller  350  is grounded. 
     (3) For the quantity of a used additive to toner, the quantity of an additive with a large particle diameter is set to 0.5 to 4.0 wt % (desirably approximately 0.7 wt %) and the quantity of an additive with a small particle diameter is set to 1.5 to 4.0 wt % (desirably approximately 2.0 wt %). 
     The reason is as described above. 
     For preventing the occurrence of a void 
     The durability of the intermediate transfer belt can be enhanced by setting the load of the secondary transfer means so that it is larger than that of the primary transfer means. This is based upon the inventors&#39; knowledge that the filming of toner to the intermediate transfer belt is caused by the additive of toner left on the intermediate transfer belt and embedded in the intermediate transfer belt by the cleaning means such as the cleaning blade for cleaning the surface of the intermediate transfer belt; the isolation of an additive often occurs in overlapping colors in order in primary transfer; since an additive which is isolated from toner and adheres to the intermediate transfer belt again adheres to relatively soft toner and a relatively soft fiber of paper as compared with the intermediate transfer belt when the above additive is pressed by a load exceeding a fixed one under toner or paper, the additive can be removed from the intermediate transfer belt. 
     Generally, the primary transfer roller  320  is always pressed on the intermediate transfer belt  360  and in the meantime, the secondary transfer roller  380  is pressed on the intermediate transfer belt  360  when a full color image in which overlapping colors is finished, is transferred. However, the secondary transfer roller is detached from the intermediate transfer belt  360  while images of each color are formed in order. However, since there occurs a phenomenon (so-called reverse transfer) in which a part of an image of the ‘n’th color is returned from the intermediate transfer belt to the photoconductive drum when an image of the (‘n’+1)th color is overlapped on the image of the ‘n’th color already formed on the intermediate transfer belt if the load of the primary transfer roller  320  is set to a load exceeding a load by which an isolated additive on the intermediate transfer belt can be removed by toner in the above constitution, it is desirable that the load of the secondary transfer roller  380  is set to a load fixed or more and in the meantime, the load of the primary transfer roller  320  is set to a load fixed or less. A load (a load required to remove an additive from the intermediate transfer belt under toner) acquired in an experiment according to the embodiment of the present invention is 150 g/cm or more and it is desirable that the above load is 200 g/cm or more. 
     To prevent reverse transfer from occurring in primary transfer, a load acquired in an experiment according to the embodiment of the present invention is 100 g/cm or less and it is desirable that the above load is 70 g/cm or less. 
     Therefore, the ratio of the respective loads of the primary transfer means and the secondary transfer means is 1.5 or more, and it is more desirable that the above ratio is 2 or more. 
     To prevent the primary and secondary transfer rollers from being bent due to a load, the shaft of each roller is required to be provided with rigidity according to the load and therefore, it is desirable that the outer diameter of the shaft of the secondary transfer roller is larger than that of the primary transfer roller. 
     According to the intermediate transfer unit of the present invention, the occurrence of a void in transfer is prevented, satisfactory transfer on rough paper can be realized and further, the durability of the intermediate transfer belt can be enhanced. 
     The following modification is also possible. 
     For preventing the occurrence of a void 
     Since resonance between the primary transfer means and the secondary transfer means can be prevented by differentiating the frequency of vibration caused by shock when the secondary transfer means comes in contact with the intermediate transfer belt from the frequency of the primary transfer means by setting the hardness of the secondary transfer roller  380  so that it is higher than the hardness of the primary transfer roller  320 , the vibration of the intermediate transfer belt and the variation of the speed respectively caused by the contact and the non-contact of the secondary transfer means with the intermediate transfer belt, can be prevented. Particularly, to reduce time required between paper and another paper and speed up the output of an image by switching the state of the secondary transfer means from the non-contact state with the intermediate transfer belt to the contact state before primary transfer is finished and starting secondary transfer, the above is very effective. It is more effective to differentiate the hardness of all rollers arranged so that each roller is touched to the intermediate transfer belt. However, in the intermediate transfer unit, the quality of a toner image on the intermediate transfer belt or the quality of a toner image on a recording medium, is mainly determined by a contact state between the primary or secondary transfer means and the intermediate transfer belt in the primary or secondary transfer position. Thus, at least by constructing as in the embodiment of the present invention, a sufficient effect can be acquired by preventing vibration in the above transfer position. 
     Further, the vibration of the intermediate transfer belt can be further satisfactorily prevented by setting the hardness of the secondary transfer roller  380  so that it is higher than the hardness of the primary transfer roller  320  by 10 degrees or more. 
     Even if a belt with a joint is used for the intermediate transfer belt, vibration caused when the primary (or the secondary) transfer means passes on the joint in the primary (or the secondary) transfer position can be prevented from being resonated by the secondary (or the primary) transfer means by setting the hardness of the secondary transfer roller  380  so that it is higher than the hardness of the primary transfer roller  320  similarly. 
     The following modification is also possible. 
     For stabilizing the efficiency of primary transfer 
     (1) A high-voltage power source which has constant-current control when the impedance of primary transfer (the ratio of the output voltage and the output current of a power source for primary transfer not shown) is large (approximately 30 MΩ or more) and has constant-voltage control when the impedance is small (approximately 30 MΩ or less), is used. The above constant current is set to 15 μA and the above constant voltage is set to 450 V. 
     Hereby, even if there is dispersion in the quantity (film thickness) of toner, environment, and the resistance of a member, satisfactory transfer is executed as shown in Table 1. 
     For comparison, Table 2 shows the result when simple constant-current control (set to 15 μA) is executed and Table 3 shows the result when simple constant-voltage control (set to 450 V) is executed. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 1 
               
               
                   
               
               
                   
                   
                 Resistance 
                   
                   
                   
               
               
                 Temperature, 
                   
                 of primary 
               
               
                 humidity &amp; 
                   
                 transfer 
                 Output 
                 Output 
                   
               
               
                 environment 
                 Printing pattern 
                 roller 
                 current 
                 voltage 
                 Result 
               
               
                   
               
             
            
               
                 10° C. 15% 
                 Printing ratio 
                 1 × 10 7  Ω 
                 15 μA 
                 700 V 
                 ◯ 
               
               
                 RH 
                 10% 
               
               
                 10° C. 15% 
                 Printing ratio 
                 1 × 10 7  Ω 
                 15 μA 
                 1000 V 
                 ◯ 
               
               
                 RH 
                 200% Solid two- 
               
               
                   
                 color overlapped 
               
               
                   
                 image 
               
               
                 23° C. 65% 
                 Printing ratio 
                 5 × 10 6  Ω 
                 30 μA 
                 450 V 
                 ◯ 
               
               
                 RH 
                 10% 
               
               
                 23° C. 65% 
                 Printing ratio 
                 5 × 10 6  Ω 
                 15 μA 
                 800 V 
                 ◯ 
               
               
                 RH 
                 200% Solid two- 
               
               
                   
                 color overlapped 
               
               
                   
                 image 
               
               
                 35° C. 65% 
                 Printing ratio 
                 3 × 10 6  Ω 
                 45 μA 
                 450 V 
                 ◯ 
               
               
                 RH 
                 10% 
               
               
                 35° C. 65% 
                 Printing ratio 
                 3 × 10 6  Ω 
                 15 μA 
                 600 V 
                 ◯ 
               
               
                 RH 
                 200% Solid two- 
               
               
                   
                 color overlapped 
               
               
                   
                 image 
               
               
                   
               
               
                 ◯: No image quality deterioration (as used hereinafter)  
               
               
                 Δ: Change is seen, however, within allowable level (as used hereinafier)  
               
               
                 x: Remarkable image quality deterioration (as used hereinafter)  
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 2 
               
               
                   
               
               
                   
                   
                 Resistance 
                   
                   
                   
               
               
                 Temperature, 
                   
                 of primary 
               
               
                 humidity &amp; 
                 Printing  
                 transfer 
                 Output 
                 Output 
                   
               
               
                 environment 
                 pattern 
                 roller 
                 current 
                 voltage 
                 Result 
               
               
                   
               
             
            
               
                 10° C. 15% 
                 Printing 
                 1 × 10 7  Ω 
                 15 μA 
                 700 V 
                 ◯ 
               
               
                 RH 
                 ratio 10% 
               
               
                 10° C. 15% 
                 Printing 
                 1 × 10 7  Ω 
                 15 μA 
                 1000 V 
                 ◯ 
               
               
                 RH 
                 ratio 200% 
               
               
                   
                 Solid two- 
               
               
                   
                 color 
               
               
                   
                 overlapped 
               
               
                   
                 image 
               
               
                 23° C. 65% 
                 Printing 
                 5 × 10 6  Ω 
                 15 μA 
                 300 V 
                 Δ 
               
               
                 RH 
                 ratio 10% 
               
               
                 23° C. 65% 
                 Printing 
                 5 × 10 6  Ω 
                 15 μA 
                 800 V 
                 ◯ 
               
               
                 RH 
                 ratio 200% 
               
               
                   
                 Solid two- 
               
               
                   
                 color 
               
               
                   
                 overlapped 
               
               
                   
                 image 
               
               
                 35° C. 65% 
                 Printing 
                 3 × 10 6  Ω 
                 15 μA 
                 150 V 
                 x 
               
               
                 RH 
                 ratio 10% 
               
               
                 35° C. 65% 
                 Printing 
                 3 × 10 6  Ω 
                 15 μA 
                 600 V 
                 ◯ 
               
               
                 RH 
                 ratio 200% 
               
               
                   
                 Solid two- 
               
               
                   
                 color 
               
               
                   
                 overlapped 
               
               
                   
                 image 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 3 
               
               
                   
               
               
                   
                   
                 Resistance 
                   
                   
                   
               
               
                 Temperature, 
                   
                 of primary 
                   
                   
                   
               
               
                 humidity &amp; 
                 Printing 
                 transfer 
                 Output 
                 Output 
               
               
                 environment 
                 pattern 
                 roller 
                 current 
                 voltage 
                 Result 
               
               
                   
               
             
            
               
                 10° C. 15% 
                 Printing 
                 1 × 10 7  Ω 
                 10 μA 
                 450 V 
                 Δ 
               
               
                 RH 
                 ratio 10% 
               
               
                 10° C. 15% 
                 Printing 
                 1 × 10 7  Ω 
                  3 μA 
                 450 V 
                 x 
               
               
                 RH 
                 ratio 200% 
               
               
                   
                 Solid two- 
               
               
                   
                 color 
               
               
                   
                 overlapped 
               
               
                   
                 image 
               
               
                 23° C. 65% 
                 Printing 
                 5 × 10 6  Ω 
                 30 μA 
                 450 V 
                 ◯ 
               
               
                 RH 
                 ratio 10% 
               
               
                 23° C. 65% 
                 Printing 
                 5 × 10 6  Ω 
                  7 μA 
                 450 V 
                 x 
               
               
                 RH 
                 ratio 200% 
               
               
                   
                 Solid two- 
               
               
                   
                 color 
               
               
                   
                 overlapped 
               
               
                   
                 image 
               
               
                 35° C. 65% 
                 Printing 
                 3 × 10 6  Ω 
                 45 μA 
                 450 V 
                 ◯ 
               
               
                 RH 
                 ratio 10% 
               
               
                 35° C. 65% 
                 Printing 
                 3 × 10 6  Ω 
                 10 μA 
                 450 V 
                 Δ 
               
               
                 RH 
                 ratio 200% 
               
               
                   
                 Solid two- 
               
               
                   
                 color 
               
               
                   
                 overlapped 
               
               
                   
                 image 
               
               
                   
               
            
           
         
       
     
     (2) The surface resistivity of the intermediate transfer belt  360  is set to 10 8  to 10 12  Ω, and the volume resistivity is set to 10 8  to 10 12  Ωcm. 
     The primary transfer roller  320  is a roller with the outer diameter of 22 mm and the width of 358 mm on a shaft 12 mm in diameter. It is made of urethane in which carbon is dispersed, the resistance is set to 10 6  to 10 8  Ω (desirably approximately 10 7  Ω), the hardness is set to 45±5°, and a load onto the photoconductive drum  110  by the primary transfer roller is set to 1.0 to 3.5 kg (desirably approximately 2.5 kg). That is, the above load is set to 28 to 98 g/cm (desirably approximately 70 g/cm). 
     Transfer is enabled at the relatively low voltage of 1200 V or less by setting the resistance value to the above range. 
     The occurrence of a so-called void can be prevented by setting the hardness and the load to the above range. 
     (3) For the quantity of a used additive to toner, the quantity of an additive with a large particle diameter (the primary particle diameter of 40 nm) is set to 0.5 to 4.0 wt % (desirably approximately 0.7 wt %) and the quantity of an additive with a small particle diameter (the primary particle diameter of 14 nm) is set to 1.5 to 4.0 wt % (desirably approximately 2.0 wt %). 
     The additive with a large particle diameter is mainly required to enhance the durable stability (the stability of the density) of toner and in view of the above, the more the quantity of the above additive is, the better it is. However, if the quantity of the above additive exceeds 4.0 wt %, the fluidity of toner is deteriorated. Thus, too much of the above additive causes the occurrence of a void, and other problems, and is not desirable. 
     In the meantime, the additive with a small particle diameter is mainly required to enhance transferability on rough paper and in view of the above, the more the quantity of the above additive is, the better it is. However, if the quantity of the above additive exceeds 4.0 wt %, the photoconductive drum  110  and the intermediate transfer belt  360  are readily filmed with floating silica so that it is not desirable. 
     For the stabilization of secondary transfer efficiency 
     (1) A high-voltage power source which has constant-current control when the impedance of secondary transfer (the ratio of the output voltage and the output current of a power source for secondary transfer not shown) is large (approximately 20 MΩ or more) and has constant-voltage control when the impedance is small (approximately 20 MΩ or less), is used. The constant current is set to 30 μA and the constant voltage is set to 600 V. 
     Hereby, even if there is dispersion in the type of paper, environment, and the resistance of a member, transfer is satisfactorily executed. 
     (2) The surface resistivity of the intermediate transfer belt  360  is set to 10 8  to 10 12  Ω, and the volume resistivity is set to 10 8  to 10 12  Ωcm. 
     The secondary transfer roller  380  is a roller with the outer diameter of 25 mm and the width of 332 mm on a shaft 15 mm in diameter. Ion conductive material such as lithium perchlorate is applied to the secondary transfer roller, the resistance is set to 10 6  to 10 8  Ω, the hardness is set to 60±5°, and a load onto the backup roller  350  by the secondary transfer roller is set to 5.0 to 9.0 kg (desirably approximately 7.0 kg). That is, the above load is set to 150 to 270 g/cm (desirably approximately 210 g/cm). 
     Transfer is enabled at 4000 V or less and 200 μA or less by setting the resistance to the above range. 
     The backup roller  350  is grounded. 
     (3) For the quantity of a used additive to toner, the quantity of an additive with a large particle diameter is set to 0.5 to 4.0 wt % (desirably approximately 0.7 wt %) and the quantity of an additive with a small particle diameter is set to 1.5 to 4.0 wt % (desirably approximately 2.0 wt %). 
     The reason is as described above. 
     According to the above conditions, the deterioration of an image due to interference in simultaneous primary and secondary transfer can be prevented and the capacity of the high-voltage power source can be reduced to the minimum. 
     As described above, according to the intermediate transfer unit of the present invention, satisfactory transferability can be secured without depending upon a printing pattern because the control of the high-voltage power source is optimized. 
     Also, transfer is enabled at required and minimum voltage and current and an imperfect image can be prevented from occurring due to abnormal discharge and others because the resistance of the primary transfer member and the intermediate transfer belt is optimized. 
     Also, the dislocation of images in primary transfer can be prevented and a phenomenon of a void can be prevented from occurring because the hardness of the primary transfer member and a load onto the photoconductive drum by the primary transfer member are optimized. 
     Also, the phenomenon of a void can be prevented from occurring because the quantity of an additive with a small particle diameter of additives added to toner is optimized and the deterioration of density due to aging can be prevented because the quantity of an additive with a large particle diameter is optimized. 
     The following modification is also possible. 
     For the stabilization of secondary transfer efficiency 
     (1) A high-voltage power source which has constant-current control when the impedance of secondary transfer (the ratio of the output voltage and the output current of a power source for secondary transfer not shown) is large (approximately 20 MΩ or more) and has constant-voltage control when the impedance is small (approximately 20 MΩ or less), is used. The constant current is set to 30 μA and the constant voltage is set to 600 V. 
     Hereby, as shown in Table 4, even if there is dispersion in the type of paper, environment, and the resistance of a member, transfer is satisfactorily executed. For comparison, Table 5 shows the result in simple constant-current control (current is set to 30 μA) and Table 6 shows the result in simple constant-voltage control (voltage is set to 600 V). 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 4 
               
               
                   
               
               
                   
                   
                 Resistance 
                   
                   
                   
               
               
                 Temperature, 
                 Type of 
                 of secondary 
                   
                   
                   
               
               
                 humidity &amp; 
                 recording 
                 transfer 
                 Output 
                 Output 
               
               
                 environment 
                 medium 
                 roller 
                 current 
                 voltage 
                 Result 
               
               
                   
               
             
            
               
                 10° C. 15% 
                 OHP sheet 
                 3 × 10 7  Ω 
                 30 μA 
                 3000 V 
                 ◯ 
               
               
                 RH 
               
               
                 10° C. 15% 
                 Xerox 4024 
                 3 × 10 7  Ω 
                 30 μA 
                 2500 V 
                 ◯ 
               
               
                 RH 
               
               
                 23° C. 65% 
                 Xerox 4024 
                 5 × 10 6  Ω 
                 30 μA 
                 800 V 
                 ◯ 
               
               
                 RH 
               
               
                 23° C. 65% 
                 Postal card 
                 5 × 10 6  Ω 
                 60 μA 
                 600 V 
                 ◯ 
               
               
                 RH 
               
               
                 35° C. 65% 
                 OHP sheet 
                 1 × 10 6  Ω 
                 30 μA 
                 1200 V 
                 ◯ 
               
               
                 RH 
               
               
                 35° C. 65% 
                 Xerox 4024 
                 1 × 10 6  Ω 
                 150 μA  
                 600 V 
                 ◯ 
               
               
                 RH 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 5 
               
               
                   
               
               
                 Temperature, 
                 Type of 
                 Resistance of 
                   
                   
                   
               
               
                 humidity &amp; 
                 recording 
                 secondary 
                 Output 
                 Output 
               
               
                 environment 
                 medium 
                 transfer roller 
                 current 
                 voltage 
                 Result 
               
               
                   
               
             
            
               
                 10° C. 15% 
                 OHP sheet 
                 3 × 10 7  Ω 
                 30 μA 
                 3000 V 
                 ◯ 
               
               
                 RH 
               
               
                 10° C. 15% 
                 Xerox 4024 
                 3 × 10 7  Ω 
                 30 μA 
                 2500 V 
                 ◯ 
               
               
                 RH 
               
               
                 23° C. 65% 
                 Xerox 4024 
                 5 × 10 6  Ω 
                 30 μA 
                 800 V 
                 ◯ 
               
               
                 RH 
               
               
                 23° C. 65% 
                 Postal card 
                 5 × 10 6  Ω 
                 30 μA 
                 300 V 
                 x 
               
               
                 RH 
               
               
                 35° C. 65% 
                 OHP sheet 
                 1 × 10 6  Ω 
                 30 μA 
                 1200 V 
                 ◯ 
               
               
                 RH 
               
               
                 35° C. 65% 
                 Xerox 4024 
                 1 × 10 6   
                 30 μA 
                 100 V 
                 x 
               
               
                 RH 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 6 
               
               
                   
               
               
                   
                   
                 Resistance 
                   
                   
                   
               
               
                 Temperature, 
                 Type of 
                 of secondary 
               
               
                 humidity &amp; 
                 recording 
                 transfer 
                 Output 
                 Output 
               
               
                 environment 
                 medium 
                 roller 
                 current 
                 voltage 
                 Result 
               
               
                   
               
             
            
               
                 10° C. 15% 
                 OHP sheet 
                 3 × 10 7  Ω 
                  5 μA 
                 600 V 
                 x 
               
               
                 RH 
               
               
                 10° C. 15% 
                 Xerox 4024 
                 3 × 10 7  Ω 
                 10 μA 
                 600 V 
                 x 
               
               
                 RH 
               
               
                 23° C. 65% 
                 Xerox 4024 
                 5 × 10 6  Ω 
                 24 μA 
                 600 V 
                 Δ 
               
               
                 RH 
               
               
                 23° C. 65% 
                 Postal card 
                 5 × 10 6  Ω 
                 60 μA 
                 600 V 
                 ◯ 
               
               
                 RH 
               
               
                 35° C. 65% 
                 OHP sheet 
                 1 × 10 6  Ω 
                 15 μA 
                 600 V 
                 x 
               
               
                 RH 
               
               
                 35° C. 65% 
                 Xerox 4024 
                 1 × 10 6  Ω 
                 150 μA  
                 600 V 
                 ◯ 
               
               
                 RH 
               
               
                   
               
            
           
         
       
     
     According to the intermediate transfer unit of the present invention, satisfactory transferability can be secured without being influenced by the type of a recording medium and environment because the control of the high-voltage power source is optimized. 
     Also, transfer is enabled at required and minimum voltage and current, and an imperfect image can be prevented from occurring due to abnormal discharge and others because the resistance of the secondary transfer member and the intermediate transfer belt is optimized. 
     Also, dislocation between images in secondary transfer can be prevented and satisfactory transfer is also enabled onto a recording medium the surface of which is rough, such as bond paper, because the hardness of the secondary transfer member and a load onto the backup roller by the secondary transfer member are optimized. 
     Also, the phenomenon of a void can be prevented from occurring because the quantity of an additive with a small particle diameter of two types of additives added to toner and different in a particle diameter is optimized and fluidity is secured, and the deterioration of density due to aging can be prevented because the quantity of an additive with a large particle diameter is optimized. 
     The following modification is also possible. 
     For the stabilization of secondary transfer efficiency 
     (1) A high-voltage power source which has constant-current control when the impedance of secondary transfer (the ratio of the output voltage and the output current of a power source for secondary transfer not shown) is large (approximately 20 MΩ or more), and has constant-voltage control when the impedance is small (approximately 20 MΩ or less), is used. The constant current is set to 30 μA and the constant voltage is set to 600 V. 
     Hereby, even if there is dispersion in the type of paper, environment, and the resistance of a member, transfer is satisfactorily executed. 
     (2) The surface resistivity of the intermediate transfer belt  360  is set to 10 8  to 10 12  Ω, and the volume resistivity is set to 10 8  to 10 12  Ωcm. 
     The secondary transfer roller  380  is a roller with the outer diameter of 25 mm and the width of 332 mm on a shaft 15 mm in diameter. Ion conductive material such as lithium perchlorate is applied to the secondary transfer roller, the resistance is set to 3×10 7  to 1×10 8  Ω in the environment of low temperature and low humidity, and set to 1×10 6  to 1×10 7  Ω in the environment of high temperature and high humidity, the hardness is set to 60±5°, and a load onto the backup roller  350  by the secondary transfer roller is set to 5.0 to 9.0 kg (desirably approximately 7.0 kg). That is, the above load is set to 150 to 270 g/cm (desirably approximately 210 g/cm). 
     Transfer is enabled at 4000 V or less and 200 μA or less by setting the resistance to the above range. 
     The backup roller  350  is grounded. 
     (3) For the quantity of a used additive to toner, the quantity of an additive with a large particle diameter is set to 0.5 to 4.0 wt % (desirably approximately 0.7 wt %), and the quantity of an additive with a small particle diameter is set to 1.5 to 4.0 wt % (desirably approximately 2.0 wt %). 
     The reason is as described above. 
     Table 7 shows an example of an experiment of the above primary transfer part and secondary transfer part. 
     
       
         
           
               
               
               
               
               
               
               
               
               
             
               
                 TABLE 7 
               
               
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                 Varia- 
                 Variation 
               
               
                   
                   
                   
                   
                   
                   
                   
                 tion of 
                 of 
               
               
                   
                   
                   
                   
                   
                   
                   
                 resis- 
                 resistance 
               
               
                   
                   
                   
                   
                   
                   
                   
                 tance 
                 due to 
               
               
                   
                   
                 Resis- 
                   
                   
                   
                   
                 due to 
                 environ- 
               
               
                   
                   
                 tance 
                 Primary 
                 Primary 
                 Resis- 
                   
                 envi- 
                 ment of 
               
               
                   
                   
                 of pri- 
                 transfer 
                 transfer 
                 tance of 
                   
                 ronment 
                 resistance 
               
               
                   
                 Temp., 
                 mary 
                 output 
                 output 
                 secon- 
                 Secon- 
                 (digit) 
                 (digit) 
               
               
                 Experi- 
                 humidity, 
                 trans- 
                 Maxi- 
                 Maxi- 
                 dary 
                 dary 
                 Primary 
                 Secondary 
               
               
                 ment 
                 environ- 
                 fer 
                 mum 
                 mum 
                 transfer 
                 transfer 
                 transfer 
                 transfer 
               
               
                 No. 
                 ment 
                 roller 
                 current 
                 voltage 
                 roller 
                 result 
                 roller 
                 roller 
               
               
                   
               
             
            
               
                 1 
                 10° C., 
                 1 × 10 7   
                 60 (μA) 
                 1200 
                 3 × 10 7   
                 Good in 
                 0.5 
                 1.5 
               
               
                   
                 15%, RH 
                 Ω 
                   
                 (V) 
                 Ω 
                 any 
               
               
                   
                   
                   
                   
                   
                   
                 paper 
               
               
                   
                   
                   
                   
                   
                   
                 type 
               
               
                 1 
                 35° C., 
                 3 × 10 6   
                 60 (μA) 
                 1200 
                 1 × 10 6   
                 Good in 
                 0.5 
                 1.5 
               
               
                   
                 65%, RH 
                 Ω 
                   
                 (V) 
                 Ω 
                 any 
               
               
                   
                   
                   
                   
                   
                   
                 paper 
               
               
                   
                   
                   
                   
                   
                   
                 type 
               
               
                 2 
                 10° C., 
                 3 × 10 7   
                 150 
                 3000 
                 1 × 10 7   
                 * 
                 1.5 
                 0.5 
               
               
                   
                 15%, RH 
                 Ω 
                 (μA) 
                 (V) 
                 Ω 
               
               
                 2 
                 35° C., 
                 1 × 10 6   
                 150 
                 3000 
                 3 × 10 6   
                 * 
                 1.5 
                 0.5 
               
               
                   
                 65%, RH 
                 Ω 
                 (μA) 
                 (V) 
                 Ω 
               
               
                   
               
               
                 “*” failure of paper transferring in small size occurs in the environment of 10° C., 15%, RH.  
               
            
           
         
       
     
     As shown in the experiment No. 1, satisfactory secondary transferability and the reduction of the capacity of the primary transfer power source can be realized by using a member having small variation of resistance due to environment for the primary transfer roller and using a member having large variation of resistance due to environment for the secondary transfer roller. 
     According to the intermediate transfer unit of the invention, since the change of the resistance of the primary transfer member and the secondary transfer member due to environment is optimized, the capacity of the primary transfer power source can be reduced and no failure of transfer in the secondary transfer part occurs both in the environment of low temperature and low humidity and in the environment of high temperature and high humidity. 
     FIG. 7 is a side view showing a modification of the intermediate transfer unit  300 . 
     In this modification, the intermediate transfer unit  300  is provided with a roller electrode  600  which is an example of the primary transfer member. Other portions in this intermediate transfer unit are the same as those in FIG.  4 . 
     The roller electrode  600  is a conductive elastic member approximately 10 mm in diameter and 5 mm in width, is located at the end of the intermediate transfer belt  360 , and is lightly in contact with the belt. Voltage is supplied to the roller electrode  600  from a high-voltage power source (not-shown) for primary transfer. 
     FIG. 8 shows an equivalent circuit in primary transfer. ‘V 1 ’ denotes the voltage of a primary transfer power source, ‘R 1 ’ denotes apparent resistance generated when a charged photoconductive drum, an intermediate transfer belt provided with a resistance layer, etc. are rotated or circulated, ‘R T ’ denotes the resistance of a primary transfer member and contact resistance, and ‘I 1 ’ denotes current for enabling primary transfer (current required for primary transfer). 
     FIG. 9 shows an equivalent circuit in case primary transfer and secondary transfer are simultaneously executed. ‘V 2 ’ denotes the voltage of a secondary transfer power source, ‘R 2 ’ denotes apparent resistance generated by a secondary transfer member and a recording medium, and ‘I 2 ’ denotes current for enabling secondary transfer (current required for secondary transfer). It is electric potential at a point A that is important in FIG.  9 . When this electric potential greatly varies, the point A is out of a suitable transfer electric field and primary transfer fails. To prevent the above failure, ‘I 2 ’ is set so that it flows on the side of the primary transfer power source by setting so that R T &lt;R 1 . Concretely, the resistance of the primary transfer member is set to 1 MΩ or less. 
     If the relationship of “I 1 &gt;I 2 ” is met under the above conditions, the failure of transfer in primary and secondary simultaneous transfer is prevented. 
     However, depending upon an environmental condition and the type of a recording medium, I 1  is smaller than I 2 . In this case, since current cannot be supplied from the primary transfer power source, electric potential at the point A is increased and transfer failure occurs. 
     “I T ” denotes the current of the primary transfer power source and under the above condition, it can be shown by an expression, I T =I 1 −I 2 . Therefore, under the condition of “I 1 &lt;I 2 ”, the current I T  of the primary transfer power source requires a function (a current absorbing function) for outputting negative current while outputting positive voltage. 
     FIG. 10 shows a case that a resistor Rx is connected in parallel to the high-voltage power source. Primary transfer power source current I TO  can be expressed by an expression “I TO =Ix+(I 1 −I 2 )” using current Ix which flows in the resistor Rx, and the above currents I 1  and I 2 . Therefore, since I TO  is positive even if “I 1 −I 2 &lt;0”, electric potential at the point A can be kept. 
     The following modification is also possible. 
     The following is related to mainly a transfer process. 
     (1) The intermediate transfer belt  360  without an end is formed by coating a sheet-shaped PET in which aluminum is deposited, with urethane paint in which PEFT particles and tin oxide as conductive material are dispersed, and by bonding both ends by ultrasonic welding. 
     Difference in a level made by bonding both ends is set to 50 μm or less and desirably set to 30 μm or less. Young&#39;s modulus of the paint is set to approximately 1.5×10 4  kgf/cm 2 . The surface resistivity of the paint is set to approximately 10 8  to 10 12  Ω, and the surface roughness is set to Rmax 1 μm (desirably 0.7 μm) or less. For the constitution of an electrode, a conductive, layer is printed on the surface of aluminum at an end, and bias is applied by the roller electrode  600  (1 MΩ or less). The primary transfer member may be also a brush, a blade, and the like except the roller electrode in this embodiment. It is important that the resistance of the primary transfer member is 1 MΩ or less. 
     The efficiency of transfer and the facility of cleaning can be enhanced by setting as described above. 
     (2) The high-voltage power source has current absorption type constant-voltage control in the primary transfer part, and applies primary transfer voltage until secondary transfer is finished. 
     The primary transfer roller (the primary transfer backup roller) functions only as a backup roller. 
     Even if secondary transfer current is larger than primary transfer current, the deterioration of the quality of an image due to interference in simultaneous primary and secondary transfer can be avoided by constituting an electrode and a power source as described above. 
     Table 8 shows the result of the above experiment. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 8 
               
               
                   
               
               
                   
                   
                   
                   
                 Image 
                 Image 
               
               
                   
                   
                   
                   
                 quality 
                 quality 
               
               
                   
                   
                   
                   
                 deterioration 
                 deterioration 
               
               
                   
                   
                   
                 Second- 
                 at 
                 at 
               
               
                   
                   
                 Primary 
                 ary 
                 simultaneous 
                 simultaneous 
               
               
                 Temp., 
                 Type of 
                 transfer 
                 transfer 
                 transfer 
                 transfer 
               
               
                 humidity, 
                 recording 
                 output 
                 output 
                 This 
                 Comparison 
               
               
                 environment 
                 medium 
                 current 
                 current 
                 embodiment 
                 example 
               
               
                   
               
             
            
               
                 10° C., 15%, 
                 OHP sheet 
                 20 μA 
                 30 μA 
                 ◯ 
                 Δ 
               
               
                 RH 
               
               
                 10° C., 15%, 
                 Xerox 
                 20 μA 
                 30 μA 
                 ◯ 
                 Δ 
               
               
                 RH 
                 4024 
               
               
                 23° C., 65%, 
                 Xerox 
                 35 μA 
                 30 μA 
                 ◯ 
                 ◯ 
               
               
                 RH 
                 4024 
               
               
                 23° C., 65%, 
                 Postal 
                 35 μA 
                 60 μA 
                 ◯ 
                 x 
               
               
                 RH 
                 card 
               
               
                 35° C., 65% 
                 OHP sheet 
                 50 μA 
                 30 μA 
                 ◯ 
                 ◯ 
               
               
                 RH 
               
               
                 35° C., 65% 
                 Xerox 
                 50 μA 
                 150 μA 
                 ◯ 
                 x 
               
               
                 RH 
                 4024 
               
               
                   
               
            
           
         
       
     
     Difference between the comparison example and this embodiment is only difference made by the high-voltage power source. 
     Heretofore, when a secondary transfer current value is larger by 10 μA or more than a primary transfer current value, the remarkable deterioration of the quality of an image occurs. However, according to the present invention, a high quality of image can be acquired independent of environment and the type of paper. 
     For stabilizing the efficiency of primary transfer 
     (1) The primary transfer high-voltage power source is set to 500 V. Current which flows during primary transfer is approximately 20 to 50 μA. 
     Since the primary transfer roller (primary transfer backup roller)  320  and the used additive to toner are the same as those in the previously described embodiment the description thereof will be omitted. 
     Further, the following modification is also possible. 
     The following description is mainly related to a transfer process: 
     (1) The intermediate transfer belt  360  without an end is formed by coating a sheet-shaped PET in which aluminum is deposited, with urethane paint in which PEFT particles and tin oxide as conductive material are dispersed, and by bonding both ends by ultrasonic welding. 
     Difference in a level made by bonding both ends is set to 50 μm or less and desirably set to 30 μm or less. Young&#39;s modulus of the paint is set to approximately 1.5×10 4  kgf/cm 2 . The surface resistivity of the paint is set to approximately 10 8  to 10 12  Ω, and the surface roughness is set to Rmax 1 μm (desirably 0.7 μm) or less. For the constitution of an electrode, a conductive layer is printed on the surface of aluminum at an end, and bias is applied by the roller electrode  600  (1 MΩ or less). The primary transfer member may be also a brush, a blade, etc. except the roller electrode in this embodiment. It is important that the resistance of the primary transfer member is 1 MΩ or less. 
     The efficiency of transfer and the facility of cleaning can be enhanced by setting as described above. 
     (2) A resistor 5 MΩ is connected in parallel to the primary transfer high-voltage power source for constant-voltage control. The primary transfer high-voltage power source applies primary transfer voltage until secondary transfer is finished. 
     The primary transfer roller (primary transfer backup roller  320 ) functions only as a backup roller. 
     Even if secondary transfer current is larger than primary transfer current, the deterioration of an image due to interference in simultaneous primary and secondary transfer can be avoided by constructing an electrode and a power source as described above. 
     Table 9 shows the result of the above experiment. 
     
       
         
           
               
               
               
               
               
               
             
               
                 TABLE 9 
               
               
                   
               
               
                   
                   
                   
                   
                 Image 
                 Image 
               
               
                   
                   
                   
                   
                 quality 
                 quality 
               
               
                   
                   
                   
                   
                 deterioration 
                 deterioration 
               
               
                   
                   
                   
                 Second- 
                 at 
                 at 
               
               
                   
                   
                 Primary 
                 ary 
                 simultaneous 
                 simultaneous 
               
               
                 Temp., 
                 Type of 
                 transfer 
                 transfer 
                 transfer 
                 transfer 
               
               
                 humidity, 
                 recording 
                 current 
                 current 
                 This 
                 Comparison 
               
               
                 environment 
                 medium 
                 I1 
                 I2 
                 embodiment 
                 example 
               
               
                   
               
             
            
               
                 10° C., 15%, 
                 OHP sheet 
                 20 μA 
                 30 μA 
                 ◯ 
                 Δ 
               
               
                 RH 
               
               
                 10° C., 15%, 
                 Xerox 
                 20 μA 
                 30 μA 
                 ◯ 
                 Δ 
               
               
                 RH 
                 4024 
               
               
                 23° C., 65%, 
                 Xerox 
                 35 μA 
                 30 μA 
                 ◯ 
                 ◯ 
               
               
                 RH 
                 4024 
               
               
                 23° C., 65%, 
                 Postal 
                 35 μA 
                 60 μA 
                 ◯ 
                 x 
               
               
                 RH 
                 card 
               
               
                 35° C., 65% 
                 OHP sheet 
                 50 μA 
                 30 μA 
                 ◯ 
                 ◯ 
               
               
                 RH 
               
               
                 35° C.,65% 
                 Xerox 
                 50 μA 
                 150 μA 
                 ◯ 
                 x 
               
               
                 RH 
                 4024 
               
               
                   
               
            
           
         
       
     
     Difference between the comparison example and this embodiment depends upon only whether a resistor is connected in parallel to the high-voltage power source or not. 
     The characters I 1  and I 2  in the table are the same as described before. 
     Heretofore, when a secondary transfer current value is larger by 10 μA or more than a primary transfer current value, the remarkable deterioration of the quality of an image occurs. However, according to the present invention, a high quality of image can be acquired independent of environment and the type of paper. 
     According to the intermediate transfer unit of the invention, since the control of the high-voltage power source is optimized and the resistance of the primary transfer member is optimized, the deterioration of the quality of an image in simultaneous primary and secondary transfer can be inhibited independent of environment and the type of paper.